Update to .NET 10, drop support for Xamarin and netstandard2.1

[tthiery]

• netstandard2.1 only serve very outdated .NET Core versions. .NET Framework was never supported with it.
• Xamarin is out of support since 2024, MAUI is available as replacement. New projects will use MAUI
• Move to .NET 10 as maintenance and new language features.

[FantasyTeddy]

Would you appreciate another PR that removes the netstandard2.1 target and updates to .NET 10?

[FantasyTeddy]

I'm also thinking about the sharpbrick/example-xamarin repository. A few ideas:

• Archive that repository and create a new one for a MAUI example.
• Re-purpose that repository and "overwrite" its content with a new MAUI example.

Let me know what you think. I could also help getting that MAUI example started.