Support .NET 11

[jonpryor]

Context: unoplatform/performance#29

.NET 11 previews are out! It would thus be nice to be able to build Uno apps targeting .NET 11, a la:

dotnet new unoapp -o myapp -preset blank -platforms android -tfm net11.0

This currently fails:

Error: Invalid option(s):
-tfm net11.0
   'net11.0' is not a valid value for -tfm. The possible values are:
      net10.0   - Target .NET 10.0
      net9.0    - Target .NET 9.0 (Standard Term Support)

It would be useful if dotnet new unoapp supported -tfm net11.0.

For which Platform:

• iOS
• Android
• WebAssembly
• WebAssembly renders for Xamarin.Forms
• Windows
• Build tasks