Migrate a Windows project to the idiomatic run loop

Flutter 2.5 replaced Windows apps' run loop with an idiomatic Windows message pump to reduce CPU usage.

Projects created before Flutter version 2.5 need to be migrated to get this improvement. You should follow the migration steps below if the windows/runner/run_loop.h file exists in your project.

Your project can be updated using these steps:

1. Verify you are on Flutter version 2.5 or newer using flutter --version
2. If needed, use flutter upgrade to update to the latest version of the Flutter SDK
3. Backup your project with git (or your preferred version control system), since you need to reapply any local changes you've made (if any) to your project in a later step
4. Delete all files under the windows/runner folder
5. Run flutter create --platforms=windows . to recreate the Windows project
6. Review the changes to files in the windows/runner folder
7. Reapply any custom changes made to the files in the windows/runner folder prior to this migration
8. Verify that your app builds using flutter build windows