4.2.1 Release checklist

[Tratcher]

• [x] Ensure there's a release branch created
• [x] Ensure the Version.props has the PreReleaseVersionLabel updated to the next version
• [x] Identify and validate the build on the CI pipeline
• [x] Release the build
• [x] Tag the commit
• [x] Publish release notes
• [x] Close the old milestone
• [x] Announce on social media
• [ ] Set the preview branch to protected
• [ ] Delete the previous preview branch
• [x] Request source code archival