Cache sidebar merge requests count at group level

Implement caching for sidebar merge requests count

Changelog: performance

app/helpers/groups_helper.rb

@@ -88,14 +88,6 @@ module GroupsHelper
       .count
   end
 
-  def group_open_merge_requests_count(group)
-    if Feature.enabled?(:cached_sidebar_merge_requests_count, group, default_enabled: :yaml)
-      cached_issuables_count(@group, type: :merge_requests)
-    else
-      number_with_delimiter(group_merge_requests_count(state: 'opened'))
-    end
-  end
-
   def group_merge_requests_count(state:)
     MergeRequestsFinder
       .new(current_user, group_id: @group.id, state: state, non_archived: true, include_subgroups: true)

app/views/layouts/nav/sidebar/_group.html.haml

 - issues_count = cached_issuables_count(@group, type: :issues)
-- merge_requests_count = group_open_merge_requests_count(@group)
+- merge_requests_count = cached_issuables_count(@group, type: :merge_requests)
 - aside_title = @group.subgroup? ? _('Subgroup navigation') : _('Group navigation')
 
 %aside.nav-sidebar{ class: ("sidebar-collapsed-desktop" if collapsed_sidebar?), **sidebar_tracking_attributes_by_object(@group), 'aria-label': aside_title }

config/feature_flags/development/cached_sidebar_merge_requests_count.yml

----
-name: cached_sidebar_merge_requests_count
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55971
-rollout_issue_url:
-milestone: '13.11'
-type: development
-group: group::product planning
-default_enabled: true

doc/user/project/merge_requests/reviews/index.md

@@ -360,32 +360,7 @@ All the above can be done with the [`git-mr`](https://gitlab.com/glensc/git-mr)
 ## Cached merge request count
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/299542) in GitLab 13.11.
-> - It's [deployed behind a feature flag](../../../feature_flags.md), enabled by default.
-> - It's enabled on GitLab.com.
-> - It's recommended for production use.
-> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-cached-merge-request-count).
-
-WARNING:
-This feature might not be available to you. Refer to the previous **version history** note for details.
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/327319) in GitLab 14.0.
 
 In a group, the sidebar displays the total count of open merge requests. This value is cached if it's greater than
 than 1000. The cached value is rounded to thousands (or millions) and updated every 24 hours.
-
-### Enable or disable cached merge request count **(FREE SELF)**
-
-Cached merge request count in the left sidebar is under development but ready for production use. It is
-deployed behind a feature flag that is **enabled by default**.
-[GitLab administrators with access to the GitLab Rails console](../../../../administration/feature_flags.md)
-can disable it.
-
-To disable it:
-
-```ruby
-Feature.disable(:cached_sidebar_merge_requests_count)
-```
-
-To enable it:
-
-```ruby
-Feature.enable(:cached_sidebar_merge_requests_count)
-```