Add warnings for WindowsDesktop targeting

[dsplaisted]

In .NET 5.0, you will need to specify the TargetPlatform of Windows in order to use Windows Forms or WPF. You also won't need to use Microsoft.NET.Sdk.WindowsDesktop, you can just use Microsoft.NET.Sdk. We should add appropriate warnings in the following situations:

• If the WindowsDesktop SDK is used, we may want to let you know that you can use Microsoft.NET.Sdk instead.
• If UseWindowsForms or UseWPF is true and the TargetFramework is net5.0 or higher (with no -windows), warn or error that the TargetFramework should be net5.0-windows to use Windows Forms or WPF
• If the TargetFramework is net5.0 or higher (with no -windows), and a transitive dependency brings in one of the Microsoft.NETCore.App.WindowsDesktop shared frameworks, we should do one or more of the following:
  • Warn or error that -windows should be added to the TargetFramework
  • Make things work: make sure the right framework is referenced in the runtimeconfig, and adjust the TargetPlatformIdentifier if necessary.