Stop caching branch_names with cache_method_as_redis_set

Context

Remove branch_names caching via RepositoryCacheAdapter (Redis Set with strong memoization). Alternative performance optimizations are being handled separately.

Implementation plan

• Create feature flag for controlled removal
• Bypass RepositoryCacheAdapter caching for branch_names when FF is enabled
• Ensure clean fallback to direct retrieval

References

• Related: Gitaly page-token pagination to protected_tags ... (#559195 - closed)
• Related: Strong memoize tag_names and branch_names in re... (#555064 - closed)
• Related: Use Gitaly to handle reference checks (!198569 - merged)