PR Summary

Allow error 504 (Gateway Timeout) in markdown-link tests

PR Context

Fix #11437

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
• User-facing changes
  • Not Applicable
• Testing - New and feature
  • N/A or can only be tested interactively
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.