Use ActionController::Base#render_to_string for view rendering

ee/app/services/merge_requests/create_from_vulnerability_data_service.rb

@@ -85,8 +85,7 @@ module MergeRequests
     end
 
     def render_template(file:, locals:)
-      view = ActionView::Base.new(ActionController::Base.view_paths, {})
-      view.render(file: file, locals: locals)
+      ActionController::Base.new.render_to_string(template: file, locals: locals)
     end
 
     def can_create_merge_request?(source_branch)