Enable post_receive_sync_refresh_cache feature flag by default

What does this MR do and why?

This feature flag has been enabled on GitLab.com for 2 weeks. It ensures that after every push, the branch and tag cache is refreshed synchronously in PostReceive to avoid race conditions when creating pipelines. However, we have seen in production that when many pushes occur that the exclusive lease can timeout. Keep the feature flag present until we confirm this doesn't cause trouble for self-managed instances.

Relates to #474741