Audit Events: Deprecate and remove the `repository_push_audit_event` feature

Problem to solve

As described in #293031, the repository_push_audit_event feature effectively duplicated the existing "push" Event type while introducing a substantial performance problem and at least one bug.

Proposal

• Deprecate the repository_push_audit_event feature in %14.3. See deprecation entry and docs update.
• Clarify the docs in %14.8. See !79933 (merged)
• Remove the repository_push_audit_event feature in %15.0.
• Close dependent issues:
  • [Feature flag] Enable logging git push events by default on gitlab.com
  • RepositoryPushAuditEventWorker throws "ActiveRecord::StatementInvalid" exception if author has no current_sign_in_ip

Out of scope

• Display push Event records in the project-level Audit Log