Enable matching_merge_request_db_sync feature flag by default

This enables by default the feature flag that was used to prevent
replication lag from causing merges to fail. It has been on for about a
week since https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44045
landed in production, and the Prometheus metrics in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44813 show that
this change has helped on occasion.

ee/config/feature_flags/development/matching_merge_request_db_sync.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42435
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/254488
 group: group::create
 type: development
-default_enabled: false
\ No newline at end of file
+default_enabled: true

ee/lib/ee/gitlab/checks/matching_merge_request.rb

@@ -11,7 +11,7 @@ module EE
 
         override :match?
         def match?
-          return super unless ::Feature.enabled?(:matching_merge_request_db_sync)
+          return super unless ::Feature.enabled?(:matching_merge_request_db_sync, default_enabled: true)
           return super unless ::Gitlab::Database::LoadBalancing.enable?
 
           # When a user merges a merge request, the following sequence happens: