Configure Dependabot for action bumps in all production branches

This repository is used to host demonstrations related to using Arduino CLI with GitHub Actions. In addition to the demo
of the "arduino/setup-arduino-cli" action hosted in the `master` branch, there is a demo of the
"arduino/compile-sketches" action in the `dependabot-all-production-branches`. This branch is a permanent and maintained
part of the repository rather than a temporary development or staging branch.

Dependabot is configured to periodically check the versions of all GitHub Actions actions used in the repository's
workflows. If any are found to be outdated, it will automatically submit a pull request to update them. These updates
are especially important in workflows that are used as a reference to the community for setting up continuous
integration of their Arduino projects.

Previously, Dependabot was only configured to provide such updates for the workflows in the `master` branch, leaving
maintenance (or lack thereof) of the workflow dependencies in the `dependabot-all-production-branches` branch to be
handled entirely manually.

Dependabot is hereby configured to also provide update PRs for the workflow dependencies in the
`dependabot-all-production-branches` branch.

Author: per1234

Diff for: .github/dependabot.yml

@@ -11,3 +11,11 @@ updates:
       interval: daily
     labels:
       - "topic: infrastructure"
+
+  - package-ecosystem: github-actions
+    target-branch: dependabot-all-production-branches
+    directory: /
+    schedule:
+      interval: daily
+    labels:
+      - "topic: infrastructure"