Introduce dedicated search_expire_finder_cache_worker feature flag
What does this MR do and why?
The existing search_finders_redis_cache FF is gated to a user actor in
Search::GroupsFinder and Search::ProjectsFinder, but was incorrectly reused
with an :instance actor in Search::ExpireFinderCacheWorker. Mixing actor
types on the same flag means enabling the flag at a % rollout (user-scoped)
doesn't reliably gate the worker.
This MR introduces a dedicated search_expire_finder_cache_worker flag
(instance-scoped) for the worker, keeping actor semantics clean and allowing
each flag to be rolled out independently.
MR acceptance checklist
- I have evaluated the MR acceptance checklist for this MR.