Add `:nested_groups` metadata to two subgroup-related specs

Prevents these from failing on MySQL.

Closes #36811 and #36812.

spec/features/explore/new_menu_spec.rb

@@ -74,7 +74,7 @@ feature 'Top Plus Menu', :js do
       expect(page).to have_content('Title')
     end
 
-    scenario 'Click on New subgroup shows new group page' do
+    scenario 'Click on New subgroup shows new group page', :nested_groups do
       visit group_path(group)
 
       click_topmenuitem("New subgroup")

spec/services/groups/create_service_spec.rb

@@ -22,7 +22,7 @@ describe Groups::CreateService, '#execute' do
     end
   end
 
-  describe 'creating subgroup' do
+  describe 'creating subgroup', :nested_groups do
     let!(:group) { create(:group) }
     let!(:service) { described_class.new(user, group_params.merge(parent_id: group.id)) }