Marked legacy ITopicRoutingService as [Obsolete]

The `ITopicRoutingService` is used by the **OnTopic-WebForms** and **OnTopic-MVC** projects as a way of determining the current topic based on the routing information. This approach was replaced with library-specific internal methods, such as **OnTopic-AspNetCore-MVC**'s `ITopicRepository` extension which allows `Load()` to accept a `RouteData` object. Further, we don't intend to provide support for this in future front-end libraries. Since **OnTopic-WebForms** is already obsolete, and **OnTopic-MVC** isn't expected to be upgraded to support **OnTopic 5.0**, we're marking this as `[Obsolete]`.

Author: JeremyCaney

OnTopic/ITopicRoutingService.cs

@@ -3,6 +3,7 @@
 | Client        Ignia, LLC
 | Project       Topics Library
 \=============================================================================================================================*/
+using System;
 
 namespace OnTopic {
 
@@ -17,6 +18,11 @@ namespace OnTopic {
   ///   adapter to framework-specific libraries (e.g., <code>HttpContext</code>). In addition, custom versions may be created
   ///   in order to establish custom mappings between URL or route data and the hierarchy of topics, should the need arise.
   /// </remarks>
+  [Obsolete(
+    "The ITopicRoutingService is no longer used in the latest clients, and will be removed in OnTopic Library 5.0. In the " +
+    "latest OnTopic.AspNetCore.Mvc libraries, it is replaced with a ITopicRepository.Load() extension method which accepts " +
+    "RouteData as an argument, in order to lookup topics based on that library's routing conventions."
+  )]
   public interface ITopicRoutingService {
 
     /*==========================================================================================================================