OnNetworkDespawn is not called on some networked game objects when the network manager is shut down

[LPLafontaineB]

Describe the bug
OnNetworkDespawn is not called on network objects when the network manager shuts down.

To Reproduce
Steps to reproduce the behavior:

1. Start a host
2. Spawn a networked gameobject
3. Shut down the NetworkManager
4. See that the object's OnNetworkDespawn event is not called.

Actual outcome
OnNetworkDespawn is not called on network objects after calling NetworkManager.Shutdown

Expected outcome
OnNetworkDespawn is called on all networked game objects

Environment (please complete the following information):

• OS: Windows 10
• Unity Version: 2020.3.13f1
• Netcode Version: 1.0.0-pre.2

Additional context
Issue encountered while working on this PR for the Boss Room sample

[0xFA11]

Tracking with MTT-1542

[JesseOlmer]

Fixed by #1390