Use linear version ApplicationSettings#elasticsearch_limited_namespaces

Changelog: performance
EE: true

config/feature_flags/development/linear_application_settings_elasticsearch_limited_namespaces.yml

+---
+name: linear_application_settings_elasticsearch_limited_namespaces
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68931
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339438
+milestone: '14.3'
+type: development
+group: group::access
+default_enabled: false

ee/app/models/ee/application_setting.rb

@@ -231,8 +231,12 @@ module EE
 
       return namespaces if ignore_descendants
 
+      if ::Feature.enabled?(:linear_application_settings_elasticsearch_limited_namespaces, default_enabled: :yaml)
+        namespaces.self_and_descendants
+      else
         ::Gitlab::ObjectHierarchy.new(namespaces).base_and_descendants
       end
+    end
 
     def pseudonymizer_available?
       License.feature_available?(:pseudonymizer)

ee/spec/models/application_setting_spec.rb

@@ -400,13 +400,13 @@ RSpec.describe ApplicationSetting do
 
         context 'with groups' do
           let(:groups) { create_list(:group, 2) }
-          let!(:indexed_namespace) { create :elasticsearch_indexed_namespace, namespace: groups.last }
+          let!(:indexed_namespace) { create(:elasticsearch_indexed_namespace, namespace: groups.last) }
+          let!(:child_group) { create(:group, parent: groups.first) }
+          let!(:child_group_indexed_through_parent) { create(:group, parent: groups.last) }
 
-          it 'returns groups that are allowed to be indexed' do
-            child_group = create(:group, parent: groups.first)
-            create :elasticsearch_indexed_namespace, namespace: child_group
-
-            child_group_indexed_through_parent = create(:group, parent: groups.last)
+          shared_examples 'returns groups that are allowed to be indexed' do
+            specify do
+              create(:elasticsearch_indexed_namespace, namespace: child_group)
 
               expect(setting.elasticsearch_limited_namespaces).to match_array(
                 [groups.last, child_group, child_group_indexed_through_parent])
@@ -415,6 +415,17 @@ RSpec.describe ApplicationSetting do
             end
           end
 
+          it_behaves_like 'returns groups that are allowed to be indexed'
+
+          context 'when feature flag :linear_application_settings_elasticsearch_limited_namespaces is disabled' do
+            before do
+              stub_feature_flags(linear_application_settings_elasticsearch_limited_namespaces: false)
+            end
+
+            it_behaves_like 'returns groups that are allowed to be indexed'
+          end
+        end
+
         describe '#elasticsearch_indexes_project?' do
           context 'when project is in a subgroup' do
             let(:root_group) { create(:group) }