move to new System.Threading.Lock type for better performance

MediaBrowser.Controller/Entities/UserRootFolder.cs

@@ -21,7 +21,7 @@ namespace MediaBrowser.Controller.Entities
     /// </summary>
     public class UserRootFolder : Folder
     {
-        private readonly object _childIdsLock = new object();
+        private readonly Lock _childIdsLock = new();
         private List<Guid> _childrenIds = null;
 
         /// <summary>