Declared SupportedOSPlatformVersion for WinAppSdk

[Arlodotexe]

Background

For net5.0+ targets, TargetPlatformMinVersion was renamed to SupportedOSPlatformVersion. See dotnet/designs#157

The dotnet SDK uses the TargetPlatformVersion property to provide a default SupportedOSPlatformVersion value if none is explicitly provided. See https://github.com/dotnet/designs/blob/bba3216250cb29b0063bac3ebb57a542ee21ad4f/accepted/2020/minimum-os-version/minimum-os-version.md?plain=1#L73C27-L73C48.

Overview

• Declares SupportedOSPlatformVersion, using the existing TargetPlatformMinVersion value.
• Fixes an issue where apps using the toolkit were no longer able to run on versions lower than 19041 (see here).
  • After spinning up VMs for Windows 10 Version 1809 Build 17763 and Windows 10 Version 21H1 Build 19043 to test the RC packages, I'm not seeing any issues building and deploying in a blank WinUI 3 app.

[Arlodotexe]

@michael-hawker do we still want to merge this for posterity?

[Sergio0694]

Why did you add this? It shouldn't be needed at all, the .NET SDK is already setting it:

This is in Microsoft.NET.Windows.targets. You should just set TargetPlatformMinVersion.

[Arlodotexe]

Why did you add this? It shouldn't be needed at all, the .NET SDK is already setting it:

For posterity, to avoid this kind of confusion in the future.

[Sergio0694]

But that confirms what I'm saying: you can just set TargetPlatformMinVersion instead, no?

[Arlodotexe]

We did, but the fact that it changed caused the confusion to start with. Docs don't specify anywhere that it's optional if TargetPlatformMinVersion is set, so we wasted time testing something that ended up being a user-side issue.