GitHub Actions: macOS virtual environment updated to Catalina

The GitHub Actions macOS virtual environment has been upgraded to Catalina (v10.15). Jobs using the `macos-10.14` virtual environment will not run and must be migrated to use `macos-latest`. In addition to Catalina, other changes include: Xcode 11.1 will be set as default Mono 6.4, Xamarin.iOS 13.4, Xamarin.Android 10.0, Xamarin.Mac 6.2 will be set as default .NET Core 3.0 will be set as default Node.JS 12 will be set

GitHub Actions: Self-hosted runners

GitHub Actions now supports self-hosted runners. Hosting your own runners offers maximum flexibility and control of your workflows. Self-host your runners if you need a proprietary image or container, access to on-prem resources, or a