Remove Classic support.

With support for Classic Xamarin.Android ended May 1st, 2024, we can start removing all the hacks needed to support both Classic and .NET for Android.

No longer build EOL frameworks: MonoAndroid12.0;net6.0-android;
No longer need to install classic XA
No longer need to have classic XA or XF sample builds
No longer need to use Xamarin.Legacy.Sdk

For this PR, we'll only build net7.0-android. A future PR should enable net8.0-android.

The best news: with this our CI build time drops from ~80 minutes to ~35 minutes. 🎉

xamarin / AndroidX

Remove Classic support. #875