Content update - Read-only dependency properties (user story 1878471) (#1230)

* Improve collection-type dp snippets that are readonly.

* Add article, snippets, toc, and redirects

* Minor edit

* Update target framework to .NET 6.0

dotnet-desktop-guide/net/wpf/properties/snippets/read-only-dependency-properties/csharp/App.xaml.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Configuration;
+using System.Data;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+    }
+}