Merge branch 'bw-fix-epic-has-children-mysql' into 'master'

Use `children.any?` in Epic since ObjectHierarchy is not supported on MySQL

Closes #11835

See merge request gitlab-org/gitlab-ee!13600

ee/app/models/ee/epic.rb

@@ -280,7 +280,7 @@ module EE
     end
 
     def has_children?
-      descendants.any?
+      children.any?
     end
 
     def has_issues?

ee/spec/models/epic_spec.rb

@@ -732,7 +732,7 @@ describe Epic do
     it 'has child epics' do
       create(:epic, group: group, parent: epic)
 
-      expect(epic.has_children?).to be_truthy
+      expect(epic.reload.has_children?).to be_truthy
     end
   end