Default-enable :update_remote_mirror_inmemory feature flag

The :update_remote_mirror_inmemory feature flag has been rolled out to
production on June 18th without any issues. Let's default-enable this
feature flag now.

Changelog: changed

config/feature_flags/development/update_remote_mirror_inmemory.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/333517
 milestone: '14.0'
 type: development
 group: group::gitaly
-default_enabled: false
+default_enabled: true