Restrict hierarchy related epic quick actions in mysql

We do not support epic hierarchy in mysql so
/child_epic and /remove_child_epic quick actions should not be
available in MySQL.

https://gitlab.com/gitlab-org/gitlab-ee/issues/11703

ee/changelogs/unreleased/11703-epic-hierarchy-quick-actions.yml

+---
+title: Restrict child_epic and remove_child_epic quick actions when using mysql
+merge_request: 13152
+author:
+type: fixed

ee/lib/ee/gitlab/quick_actions/epic_actions.rb

@@ -51,7 +51,7 @@ module EE
           end
 
           def action_allowed?
-            quick_action_target.group&.feature_available?(:epics) &&
+            ::Epic.supports_nested_objects? && quick_action_target.group&.feature_available?(:epics) &&
               current_user.can?(:"admin_#{quick_action_target.to_ability_name}", quick_action_target)
           end
         end

ee/spec/services/groups/autocomplete_service_spec.rb

@@ -101,10 +101,10 @@ describe Groups::AutocompleteService do
       end
 
       it 'returns available commands' do
-        expect(subject.commands(epic).map { |c| c[:name] })
-          .to match_array(
-            [:todo, :unsubscribe, :award, :shrug, :tableflip, :cc, :title, :close, :child_epic, :remove_child_epic]
-          )
+        available_commands = [:todo, :unsubscribe, :award, :shrug, :tableflip, :cc, :title, :close]
+        available_commands += [:child_epic, :remove_child_epic] if ::Epic.supports_nested_objects?
+
+        expect(subject.commands(epic).map { |c| c[:name] }).to match_array(available_commands)
       end
     end
   end

ee/spec/services/quick_actions/interpret_service_spec.rb

@@ -807,7 +807,7 @@ describe QuickActions::InterpretService do
         group.add_developer(current_user)
       end
 
-      context 'child_epic command' do
+      context 'child_epic command', :nested_groups do
         context 'when correct epic reference' do
           let(:content) { "/child_epic #{child_epic&.to_reference(epic)}" }
 
@@ -827,7 +827,7 @@ describe QuickActions::InterpretService do
         end
       end
 
-      context 'remove_child_epic command' do
+      context 'remove_child_epic command', :nested_groups do
         context 'when correct epic reference' do
           let(:content) { "/remove_child_epic #{child_epic&.to_reference(epic)}" }