Fix Approval UI showing up for free plan

What does this MR do?

Approval related UI are showing for free plan projects in

• MR edit page
• MR show page widget

This MR hides those UI elements for free plan projects.

What are the relevant issue numbers?

Closes #9908 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

ee/app/models/concerns/approvable.rb

@@ -8,7 +8,7 @@ module Approvable
   include ::VisibleApprovable
 
   def approval_feature_available?
-    strong_memoize(:approval_feature_available?) do
+    strong_memoize(:approval_feature_available) do
       project.feature_available?(:merge_request_approvers)
     end
   end