Update all projects to use .NET 6 where available

[Arlodotexe]

Describe the bug

The following projects need to be updated to use .NET 6:

• [ ] WPF
• [ ] Android
• [ ] iOS
• [ ] MacOS
• [ ] Gtk (linux)
• [ ] WASM

This will require generating a blank Uno project and pulling in the new Heads, adapting each one to be usable within the Labs infrastructure.

Additional context

No response

Help us help you

Yes, but only if others can assist.

[Arlodotexe]

The WPF head was removed from the "all" solution in #258 due to various issues with Uno/WPF/MSBuild.

A few tickets have been filed, once we get a buildable WPF head again we can add it back (related to #125)

• https://github.com/unoplatform/uno/issues/9423
• https://github.com/unoplatform/uno/issues/9380

[Arlodotexe]

Visual Studio 17.3 just came out of preview, and comes preinstalled with the .NET 6 MAUI workloads required by Uno.

https://devblogs.microsoft.com/visualstudio/visual-studio-2022-17-3-is-now-available/