Quarantine user_uploads_designs_spec.rb:21

What does this MR do and why?

This MR quarantines spec/features/projects/issues/design_management/user_uploads_designs_spec.rb:21 until we find out the root cause of the flakiness.

Related to #358845 and gitlab-org/quality/engineering-productivity/master-broken-incidents#525 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.