Update and dequarantine branch_with_unusual_name E2E

What does this MR do?

Dequarantines branch_with_unusual_name_spec that was quarantined due to the bug described in #364565 (closed) (now fixed). While in quarantine, the test got stale due to migration to Pajamas component for dropdown used in branch selection, this is also addressed by using a generic Dropdown component in page object and unused qa-selectors are removed.

Part of gitlab-org/quality/quality-engineering/team-tasks#1621 (closed) efforts.

E2E Test Failure issue(s)

#383863 (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).
• Dequarantine test check-list
  • Follow the Dequarantining Tests guide.
  • Confirm the test consistently passes on the target GitLab environment(s).
• To ensure a faster turnaround, ask in the #quality_maintainers Slack channel for someone to review and merge the merge request, rather than assigning it directly.