Quarantine a flaky test

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/features/merge_request/user_sees_diff_spec.rb

@@ -69,7 +69,7 @@ RSpec.describe 'Merge request > User sees diff', :js do
     end
 
     context 'as user who needs to fork' do
-      it 'shows fork/cancel confirmation', :sidekiq_might_not_need_inline do
+      it 'shows fork/cancel confirmation', :sidekiq_might_not_need_inline, quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/337477' do
         sign_in(user)
         visit diffs_project_merge_request_path(project, merge_request)