Use Dependabot to check for outdated action versions used in workflows

.github/dependabot.yml

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+# See: https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates#about-the-dependabotyml-file
+version: 2
+
+updates:
+  # Configure check for outdated GitHub Actions actions in workflows.
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/dependabot/README.md
+  # See: https://docs.github.com/en/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+  - package-ecosystem: github-actions
+    directory: / # Check the repository's workflows under /.github/workflows/
+    schedule:
+      interval: daily
+    labels:
+      - "topic: infrastructure"
+    assignees:
+      - per1234
+
+  # Configure check for outdated GitHub Actions actions in workflow templates.
+  - package-ecosystem: github-actions
+    # The workflows under the .github/workflows/ subfolder of this path will be checked.
+    directory: /workflow-templates/dependabot/workflow-template-copies/
+    schedule:
+      interval: daily
+    commit-message:
+      prefix: (DO NOT MERGE)
+    labels:
+      - "topic: infrastructure"
+    assignees:
+      - per1234

.github/workflows/check-sync.yml

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+# This repository contains intentionally duplicated copies of files:
+# - Workflow template copies under workflow-templates/dependabot/workflow-template-copies used for Dependabot checks.
+# - Workflow template copies in .github/workflows used for this repository's own CI system.
+# - Assets for the workflow templates used in this repository.
+#
+# This workflow checks that the copies are in sync.
+# If the workflow fails, run workflow-templates/etc/sync.sh and commit.
+
+name: Check File Duplicates Sync
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  check-sync:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Check file duplicates sync
+        run: |
+          SYNC_SCRIPT_PATH="etc/sync.sh"
+          "${{ github.workspace }}/$SYNC_SCRIPT_PATH"
+          git add .
+          if ! git diff --color --exit-code HEAD; then
+            echo "::error::File duplicates are out of sync. Please run $SYNC_SCRIPT_PATH"
+            exit 1
+          fi

etc/sync.sh

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+#!/bin/bash
+# Sync the repository's intentionally duplicated files.
+
+readonly REPOSITORY_ROOT_PATH="$(git rev-parse --show-toplevel)"
+readonly WORKFLOW_TEMPLATE_ASSETS_PATH="${REPOSITORY_ROOT_PATH}/workflow-templates/assets"
+readonly WORKFLOW_TEMPLATES_PATH="${REPOSITORY_ROOT_PATH}/workflow-templates"
+readonly WORKFLOWS_PATH="${REPOSITORY_ROOT_PATH}/.github/workflows"
+readonly WORKFLOW_TEMPLATE_COPIES_PATH="${REPOSITORY_ROOT_PATH}/workflow-templates/dependabot/workflow-template-copies/.github/workflows"
+
+# Sync workflow templates with the copies in the folder where Dependabot can check them for updates.
+mkdir --parents "$WORKFLOW_TEMPLATE_COPIES_PATH"
+rm --force "${WORKFLOW_TEMPLATE_COPIES_PATH}/"*
+find "$WORKFLOW_TEMPLATES_PATH" -maxdepth 1 -type f -and \( -name '*.yml' -or -name '*.yaml' \) -exec cp '{}' "$WORKFLOW_TEMPLATE_COPIES_PATH" \;

workflow-templates/assets/dependabot/README.md

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+# Dependabot for GitHub Actions
+
+Dependabot can be used to check for outdated action versions used in the repository's GitHub Actions workflows:
+
+https://docs.github.com/en/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+
+## Instructions
+
+Just copy [this `dependabot.yml` file](dependabot.yml) to the `.github/` folder of the target repository (or add the entry if the repository already has a `/.github/dependabot.yml`) and everything else is handled automatically.
+
+### Note
+
+Dependabot's PRs will occasionally propose to pin to the patch version of the action (e.g., updating `uses: foo/bar@v1` to `uses: foo/[email protected]`). When the action author has [provided a major version ref](https://docs.github.com/en/actions/creating-actions/about-actions#using-release-management-for-actions), use that instead (e.g., `uses: foo/bar@v2`). Once the major version has been updated in the workflow, Dependabot should not submit an update PR again until the next major version bump.
+
+So even when the PRs from Dependabot are not exactly correct, they still have value in bringing the maintainer's attention to the fact that the action version in use is outdated. The effort needed to manually adjust the ref when this happens is trivial.
+
+Dependabot will automatically close its PR once the workflow has been updated.
+
+## Commit message
+
+```
+Configure Dependabot to check for outdated actions used in workflows
+
+Dependabot will periodically check the versions of all actions used in the repository's workflows. If any are found to
+be outdated, it will submit a pull request to update them.
+
+NOTE: Dependabot's PRs will occasionally propose to pin to the patch version of the action (e.g., updating
+`uses: foo/bar@v1` to `uses: foo/[email protected]`). When the action author has provided a major version ref, use that instead
+(e.g., `uses: foo/bar@v2`). Dependabot will automatically close its PR once the workflow has been updated.
+
+More information:
+https://docs.github.com/en/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+```
+
+## PR message
+
+```Markdown
+Dependabot will periodically check the versions of all actions used in the repository's workflows. If any are found to be outdated, it will submit a pull request to update them.
+
+NOTE: Dependabot's PRs will occasionally propose to pin to the patch version of the action (e.g., updating `uses: foo/bar@v1` to `uses: foo/[email protected]`). When the action author has [provided a major version ref](https://docs.github.com/en/actions/creating-actions/about-actions#using-release-management-for-actions), use that instead (e.g., `uses: foo/bar@v2`). Once the major version has been updated in the workflow, Dependabot should not submit an update PR again until the next major version bump.
+
+So even when the PRs from Dependabot are not exactly correct, they still have value in bringing the maintainer's attention to the fact that the action version in use is outdated. Dependabot will automatically close its PR once the workflow has been updated.
+
+More information:
+https://docs.github.com/en/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+```

workflow-templates/assets/dependabot/dependabot.yml

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+# See: https://docs.github.com/en/code-security/supply-chain-security/configuration-options-for-dependency-updates#about-the-dependabotyml-file
+version: 2
+
+updates:
+  # Configure check for outdated GitHub Actions actions in workflows.
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/dependabot/README.md
+  # See: https://docs.github.com/en/code-security/supply-chain-security/keeping-your-actions-up-to-date-with-dependabot
+  - package-ecosystem: github-actions
+    directory: / # Check the repository's workflows under /.github/workflows/
+    schedule:
+      interval: daily
+    labels:
+      - "topic: infrastructure"