Only expire branch cache once per push

What does this MR do?

Whenever PostReceive is enqueued, UpdateMergeRequestsWorker is enqueued and MergeRequests::RefreshService is called, it'll check if the source branch of each MR asssociated to the push exists or not via MergeRequest#source_branch_exists?. The said method will call Repository#branch_exists? which is cached in Rails.cache.

When the cache contains outdated data and the source branch actually exists, the MergeRequests#RefreshService job will close associated MRs which is not correct.

The fix is to expire the branches cache of the project so we have updated data during the post receive hook which will help in the accuracy of the check if we need to close associated MRs or not.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#65803 (closed)