chore: add grpc-experimental branch rules

[cindy-peng]

Enable github branch rules for gapic client branch grpc-experimental.
We need the dependencies to be up-to-date before launching the public preview.

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Fixes #<issue_number_goes_here> ☕️

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/sync-repo-settings.yaml