Quarantine praefect_repo_sync E2E spec

What does this MR do?

Quarantines qa/specs/features/api/3_create/gitaly/praefect_repo_sync_spec.rb which is failing consistently. The failure cause is being investigated in #347415 (comment 757346321), it appears it's related to recent code change with Gitaly but needs t be confirmed.

E2E Test Failure issue(s)

#347415 (closed)

Check-list

• General code guidelines check-list
  • Code review guidelines
  • Style guides
• Quarantine test check-list
  • Follow the Quarantining Tests guide.
  • Confirm the test has a quarantine: tag with the specified quarantine type.
  • Note if the test should be quarantined for a specific environment.
  • (Optionally) In case of an emergency (e.g. blocked deployments), consider adding labels to pick into auto-deploy (Pick into auto-deploy priority1 severity1).