fix: Server was able to be mapped as both ID 0 and ID 1

[ShadauxCat]

This meant the first client to connect could sometimes see messages from the server as coming from itself, and could successfully send to its own LocalClientId (even though the message would go to the server). Other clients would be able to send and receive from id 1, but that would also be the server.

MTT-1536
MTT-1691

Changelog

com.unity.netcode.gameobjects

• Fixed: Fixed an issue where sometimes the first client to connect to the server could see messages from the server as coming from itself.
• Fixed: Fixed an issue where clients seemed to be able to send messages to ClientId 1, but these messages would actually still go to the server (id 0) instead of that client.
• Fixed: Improved clarity of error messaging when a client attempts to send a message to a destination other than the server, which isn't allowed.

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary.

[josiemessa]

Have tested this out on my test project, and can now see messages being received from the server when we profile the client. I am now getting spammed with these errors but I think that's OK as the code I was using to test this is now invalid:

ArgumentException: Clients may only send messages to ServerClientId
Unity.Netcode.NetworkManager.SendMessage[T] (T& message, Unity.Netcode.NetworkDelivery delivery, System.UInt64 clientId) (at C:/dev/src/github.com/Unity-Technologies/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs:1517)
Unity.Netcode.CustomMessagingManager.SendNamedMessage (System.String messageName, System.UInt64 clientId, Unity.Netcode.FastBufferWriter messageStream, Unity.Netcode.NetworkDelivery networkDelivery) (at C:/dev/src/github.com/Unity-Technologies/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Messaging/CustomMessageManager.cs:228)
Player.Update () (at Assets/Scripts/Network/Player.cs:54)

[ShadauxCat]

Reopened as #1683