fix: Remove explicit type in Solution template App.xaml.cs

- In case Uno Platform solution is created in a #nullable context, the app is not buildable because as keyword makes the Frame nullable. Using var this issue is avoided and automatically becomes Frame? in nullable.

src/SolutionTemplate/UnoSolutionTemplate/Shared/App.xaml.cs

@@ -56,7 +56,7 @@ protected override void OnLaunched(LaunchActivatedEventArgs e)
 			var window = Windows.UI.Xaml.Window.Current;
 #endif
 
-			Frame rootFrame = window.Content as Frame;
+			var rootFrame = window.Content as Frame;
 
 			// Do not repeat app initialization when the Window already has content,
 			// just ensure that the window is active