Remove Classic support.

[jpobst]

Context: https://github.com/xamarin/AndroidX/pull/875

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.