Obsolete Clients property on HttpConnection (#4801) (#4802)

Relates: #4673

This commit adds the Clients property back to HttpConnection
,for binary compatibility, with an ObsoleteAttribute applied
to indicate that the property is no longer used.

Co-authored-by: Russ Cam <russ.cam@elastic.co>

Author: github-actions[bot]

src/Elasticsearch.Net/Connection/HttpConnection.cs

@@ -46,9 +46,12 @@ public class HttpConnection : IConnection
 			+ $" please set {nameof(ConnectionConfiguration.ConnectionLimit)} to -1 on your connection configuration/settings."
 			+ $" this will cause the {nameof(HttpClientHandler.MaxConnectionsPerServer)} not to be set on {nameof(HttpClientHandler)}";
 
-		private readonly object _lock = new object();
 		private RequestDataHttpClientFactory HttpClientFactory { get; }
 
+		[Obsolete("HttpConnection now uses a HttpClientFactory implementation to manage HttpClient and HttpMessageHandler instances. "
+			+ "This property is no longer used and will be removed in the next major release")]
+		protected readonly ConcurrentDictionary<int, HttpClient> Clients = new ConcurrentDictionary<int, HttpClient>();
+
 		public int InUseHandlers => HttpClientFactory.InUseHandlers;
 		public int RemovedHandlers => HttpClientFactory.RemovedHandlers;