Fix caching logic for protected branches

• Bug report: gitlab-com/gl-infra/scalability#1601 (comment 1010867339)
• Hotfix: !91410 (merged)

Problem

Protected branches cache depends on a project cache key. But the cache key updates for every new issue, merge request and push to the project. That triggers a creation of new protected branches cache without removing the old versions of it.

As a result, we generate lots of cache keys that get stale quickly.

Proposal

• Use a better strategy for caching. As an option, we can refresh the cache only when protected branches are changed.
• Clean-up stale cache keys

Feature flags

• hash_based_cache_for_protected_branches - enables the new caching implementation (but by default it's in dry-run mode)
• rely_on_protected_branches_cache - requires hash_based_cache_for_protected_branches to be enabled and controls the dry-run mode.

hash_based_cache_for_protected_branches	rely_on_protected_branches_cache	Result
false	false	Use the old cache
false	true	Use the old cache
true	false	Use the new cache (dry-run: true)
true	true	Use the new cache (dry-run: false)