Enable caching on merge requests API

Changelog: performance

config/feature_flags/development/api_caching_merge_requests.yml

----
-name: api_caching_merge_requests
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61067
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330342
-milestone: '13.12'
-type: development
-group: group::source code
-default_enabled: false

lib/api/merge_requests.rb

@@ -207,11 +207,7 @@ module API
         options = serializer_options_for(merge_requests).merge(project: user_project)
         options[:project] = user_project
 
-        if Feature.enabled?(:api_caching_merge_requests, user_project, type: :development, default_enabled: :yaml)
-          present_cached merge_requests, expires_in: 2.days, **options
-        else
-          present merge_requests, options
-        end
+        present_cached merge_requests, expires_in: 2.days, **options
       end
 
       desc 'Create a merge request' do

spec/requests/api/merge_requests_spec.rb

@@ -1005,14 +1005,6 @@ RSpec.describe API::MergeRequests do
 
     it_behaves_like 'merge requests list'
 
-    context 'when :api_caching_merge_requests is disabled' do
-      before do
-        stub_feature_flags(api_caching_merge_requests: false)
-      end
-
-      it_behaves_like 'merge requests list'
-    end
-
     it "returns 404 for non public projects" do
       project = create(:project, :private)