chore: exclude bots from CONTRIBUTORS file (#26855)

Exclude known bots. The created list isn't amazing yet, it's mostly a list of CDK Core team members.
Will have to do some more iterations in future.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

.github/workflows/update-contributors.yml

@@ -16,6 +16,7 @@ jobs:
         env:
           CONTRIB_REPOSITORY: 'aws/aws-cdk'
           CONTRIB_OUTPUT_FILE: 'CONTRIBUTORS.md'
+          CONTRIB_IGNORE: 'github-actions[bot],mergify[bot],dependabot[bot],dependabot-preview[bot],aws-cdk-automation'
       - name: Create a PR
         uses: peter-evans/create-pull-request@v5
         with: