Content update - Add attached property snippet tests (user story 1878471) (#1224)

* Add snippet test and update type

* Add snippet test and update type

* Update to .NET to 6.0

* Update accessor signatures

dotnet-desktop-guide/net/wpf/properties/snippets/how-to-register-an-attached-property/vb/MainWindow.xaml.vb

@@ -8,13 +8,19 @@
 
         Public Sub New()
             InitializeComponent()
+
+            ' Test get/set accessors.
+            Dim aquarium As New Aquarium()
+            Aquarium.SetHasFish(aquarium, True)
+            Dim hasFish As Boolean = Aquarium.GetHasFish(aquarium)
+            Debug.WriteLine($"Has fish: {hasFish}")
         End Sub
 
     End Class
 
     '<RegisterAttachedProperty>
-    Public Class Aquarium 
-        Inherits DependencyObject
+    Public Class Aquarium
+        Inherits UIElement
 
         ' Register an attached dependency property with the specified
         ' property name, property type, owner type, and property metadata.