fix: networkprefab upgrade with formerlyserializedas attribute (#619)

* fix: Add formerly serialized as attribute

* revert: unecessary transport attribute

Author: LukeStampfli

com.unity.multiplayer.mlapi/Runtime/Configuration/NetworkConfig.cs

@@ -6,6 +6,7 @@
 using MLAPI.Hashing;
 using MLAPI.Serialization;
 using MLAPI.Serialization.Pooled;
+using UnityEngine.Serialization;
 
 namespace MLAPI.Configuration
 {
@@ -52,6 +53,7 @@ public class NetworkConfig
         /// A list of spawnable prefabs
         /// </summary>
         [Tooltip("The prefabs that can be spawned across the network")]
+        [FormerlySerializedAs("NetworkedPrefabs")]
         public List<NetworkPrefab> NetworkPrefabs = new List<NetworkPrefab>();
 
         /// <summary>