Fix several warnings

[pauldendulk]

No description provided.

[pauldendulk]

Done:

• Avalonia to 11.0.6. This fixed warnings on Android
• All samples and tests to .NET 8, this fixed several warnings
• Add <Nullable>enable</Nullable> to the Avalonia specific Directory.Build.props to fix null annotation warnings (which should not have been there).
• Remove duplicate references for Microsoft.AspNetCore.Components.Web*
• Return the instance instead of an abstraction from a method in two places
• Use await cancelationTokenSource.CancelAsync() instead of cancelationTokenSource.Cancel() in method returning Task.
• Suppress a Warning CA1416 (Call site reachable by all platforms) in the WPF samples
• Made a method static following suggestion.
• Add script delete bin and obj