Avoiding breaking change

Author: noellie-velez

com.unity.netcode.gameobjects/Runtime/Core/NetworkObject.cs

@@ -353,7 +353,7 @@ internal bool HasParentNetworkObject(Transform transform)
         /// <summary>
         /// Gets the NetworkManager that owns this NetworkObject instance.
         /// </summary>
-        public NetworkManager NetworkManager = NetworkManager.Singleton;
+        public NetworkManager NetworkManager { get; private set; } = NetworkManager.Singleton;
 
         /// <summary>
         /// Useful to know if we should or should not send a message