Marking .NET PageFactory obsolete

This implementation of PageFactory for .NET is deeply flawed.
Additionally, using the PageFactory provides no benefit over other methods
of Page Object creation in .NET. This is true for code verbosity as well,
which is often the reason cited for wanting to use the PageFactory in
.NET. The existing code has been migrated to a new repository under a new
organization on GitHub
(https://github.com/DotNetSeleniumTools/DotNetSeleniumExtras). It is hoped
that this will encourage a volunteer from the community to take ownership
of this code. Users should update their references and migrate their code
to use `SeleniumExtras.PageFactory`. The implementation will be removed
from the .NET language bindings in a future release.

dotnet/src/support/PageObjects/PageFactory.cs

@@ -26,6 +26,7 @@ namespace OpenQA.Selenium.Support.PageObjects
     /// <summary>
     /// Provides the ability to produce Page Objects modeling a page. This class cannot be inherited.
     /// </summary>
+    [Obsolete("The PageFactory implementation in the .NET bindings is deprecated and will be removed in a future release. This portion of the code has been migrated to the DotNetSeleniumExtras repository on GitHub (https://github.com/DotNetSeleniumTools/DotNetSeleniumExtras)")]
     public sealed class PageFactory
     {
         /// <summary>