Update build agent per requirement

[daxian-dbw]

PR Summary

Update build agent per requirement.
It's required to update the build agent to use MMSWindows2019-Secure. Test build: https://dev.azure.com/mscodehub/PSReadLine/_build/results?buildId=235174&view=results (build failed as expected, but the build agent works)

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow