.NET 6: finish templates for GA

[jonathanpeppers]

The basic templates for xamarin-android are implemented in #5348.

This got us basic templates for .NET 6 Preview 1.

For .NET 6 GA, we would potentially need more changes:

1. Every template in VS today needs an equivalent template for .NET 6: unless we decide to drop some of them.
2. We might be able to make use of the new MSBuild project properties and omit values in AndroidManifest.xml

Equivalent issue on the xamarin-macios side: dotnet/macios#10747

[jonathanpeppers]

Closing this in favor of: #6076