Update WinUICs Default Template to use new System Backdrop APIs (#4660)

code/TemplateStudioForWinUICs/Templates/Proj/Default/Param_ProjectName/MainWindow.xaml

@@ -10,7 +10,7 @@
     MinHeight="500"
     PersistenceId="MainWindow"
     mc:Ignorable="d">
-    <windowex:WindowEx.Backdrop>
-        <windowex:MicaSystemBackdrop/>
-    </windowex:WindowEx.Backdrop>
+    <Window.SystemBackdrop>
+        <MicaBackdrop/>
+    </Window.SystemBackdrop>
 </windowex:WindowEx>