Address CodeQL: use System.Threading.Lock instead of object for WebSocketServer.syncRoot

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/c8ad2ddc-3e69-424b-aa69-46b457a9df9b

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game/IPC/WebSocketServer.cs

@@ -61,7 +61,7 @@ public sealed class WebSocketServer : IDisposable
         /// </summary>
         public event Action<int, string>? MessageReceived;
 
-        private readonly object syncRoot = new object();
+        private readonly Lock syncRoot = new Lock();
 
         private readonly string prefix;
         private readonly Logger logger;