Enable optimized merged at count FF by default

This change enables the
`optimized_merge_request_count_with_merged_at_filter` feature flag by
default.

app/graphql/resolvers/project_merge_requests_resolver.rb

@@ -10,7 +10,7 @@ module Resolvers
     def resolve(**args)
       scope = super
 
-      if only_count_is_selected_with_merged_at_filter?(args) && Feature.enabled?(:optimized_merge_request_count_with_merged_at_filter)
+      if only_count_is_selected_with_merged_at_filter?(args) && Feature.enabled?(:optimized_merge_request_count_with_merged_at_filter, default_enabled: :yaml)
         MergeRequest::MetricsFinder
           .new(current_user, args.merge(target_project: project))
           .execute

config/feature_flags/development/optimized_merge_request_count_with_merged_at_filter.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299347
 milestone: '13.9'
 type: development
 group: group::optimize
-default_enabled: false
+default_enabled: true

ee/changelogs/unreleased/299347-enable-optimized_merge_request_count_with_merged_at_filter-ff.yml

+---
+title: Improve the performance of the Merge Request Analytics chart
+merge_request: 53704
+author:
+type: performance