Dequarantine file_locking_spec test

What does this MR do?

Dequarantines 'creates a merge request and fails to merge' example from qa/specs/features/ee/browser_ui/3_create/repository/file_locking_spec.rb E2E test. The UX bug it was waiting on has been fixed.

E2E Test Failure issue(s)

See #40125 (closed)

Check-list

• General code guidelines check-list
  • Code review guidelines
  • Style guides
• Dequarantine test check-list
  • Follow the Dequarantining Tests guide.
  • Confirm the test consistently passes on the target GitLab environment(s).
    • [-] (Optionally) Trigger a manual GitLab-QA pipeline against a specific GitLab environment using the RELEASE variable from the package-and-qa job of the current merge request.
• To ensure a faster turnaround, ask in the #quality Slack channel for someone to review and merge the merge request, rather than assigning it directly.