Mark kestrel server as obsolete

Author: jkotalik

src/Servers/Kestrel/Core/src/KestrelServer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -13,6 +14,7 @@
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Core
 {
+    [Obsolete("This type is obsolete and will be removed in a future version.")]
     public class KestrelServer : IServer
     {
         private KestrelServerImpl _innerKestrelServer;