Deprecate the WindowsAppSDK app template and projects

[jonthysell]

Summary

As we work on the plan-of-record Fabric implementation based on Composition, we no longer need the experimental projects based on the Windows App SDK.

Specifically, we need to:

• Remove the existing WindowsAppSDK code from react-native-windows:
  • ./vnext/Microsoft.ReactNative.WindowsAppSDK.sln solution
  • ./vnext/Microsoft.ReactNative.WindowsAppSDK/ directory and projects
  • ./vnext/template/cs-app-WinAppSDK/ template
  • ./vnext/PropertySheets/External/Microsoft.ReactNative.WinAppSDK.*.* property sheets
  • ./vnext/Scripts/Microsoft.ReactNative.WindowsAppSDK.nuspec nuget config file
• Remove new project code from @react-native-windows/cli
• Remove WinUI3 permutations (i.e. RnwNativeBuildWinAppSDK job) from ./ado/publish.yml
• Remove WinUI3 permutations from matrix in ./ado/jobs/cli-init.yml
• Remove ./ado/jobs/windowsappsdk.yml
• Remove WinAppSDK permutations from ./ado/stages.yml

Motivation

These projects/templates were early, experimental work for a future of RNW apps being WinAppSDK-based. Since we're not going in that direction, we shouldn't persist allowing projects to be created using it.

Basic Example

No response

Open Questions

• What happens if we leave the UseWinUI3 flag in our other projects? Without a published binary, is that flag even useful anymore?
  • Maybe for "unsupported" UWP+WinUI3 apps?
  • Maybe for unpackaged win32 apps like playground-win32?

[jonthysell]

Possibly related to #8133?