Make stable branch commit check clearer

[hqpho]

Summary

• Include remediation instructions in error message.
• Rename check file to "Release branch checks" (release-branch-checks.yml)
  • This makes the prefix of the check where it's listed on a PR non-redundant, and also sets us up to potentially have more checks in there in the future.
  • I've called it "Release branch checks" instead of "Custom DC stable branch checks" because technically the same checks would apply if we ever wanted to use release branches for website prod and staging.
• Rename job to "verify_all_commits_are_already_in_master" so the name makes sense on its own.

Also, update version of checkout action to one that isn't about to be deprecated.

Screenshots

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