E2E: Dequarantine purchase_ci_spec

What does this MR do?

The original bug HTTP Status Code 500 in GitLab Checkout | Subscription not found that was causing the intermittent failures in #439500 (closed) appears to have been resolved (see this discussion)

This MR dequarantines ee/browser_ui/11_fulfillment/purchase/purchase_ci_spec.rb which will also help us continue monitoring the bug fix implemented above

E2E Test Failure issue(s)

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).
• 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.