Replace UWP with WinUI (#49672)

The Windows docs team wants to emphasize WinUI over UWP

Author: BillWagner

docs/index.yml

@@ -271,8 +271,8 @@ additionalContent:
         # Card
         - title: Desktop
           links:
-            - url: /uwp
-              text: Universal Windows apps
+            - url: /windows/apps/winui
+              text: WinUI apps
             - url: /dotnet/desktop/wpf/
               text: Windows Presentation Foundation
             - url: /dotnet/desktop/winforms/