fix: ObjectDisposedException on Unity Transport shutdown [backport]

[EmandM]

Ensures that UnityTransport is marked as Disconnected after shutting down. This ensures that no exceptions are thrown when trying to use the transport after it has been shut down.

This PR also adds a warning message when a UnityTransport has a NetworkManager and UnityTransport.Shutdown() has been called outside of NetworkManager.Shutdown() as shutting down the UnityTransport directly will result in unexpected disconnection behaviour (connected clients will not get Disconnect events and pending messages may be dropped).

fix: #3118

Changelog

• Fixed: Issue where an ObjectDisposedException was thrown when calling NetworkManager.Shutdown() after calling UnityTransport.Shutdown()

Testing and Documentation

• Unit tests have been added to UnityTransportTests to ensure exceptions are not thrown when using the transport after shutting it down.
• No documentation changes or additions were necessary.

[NoelStephensUnity]