fix: #2802 NetworkManager uses OnDestroy for shutdown instead of OnApplicationQuit

[miwarnec]

#2802

[imerr]

Transport still has the same issue, it currently shuts down in OnApplicationQuit, that'd need to be covered too to fix the issue

Mirror/Assets/Mirror/Core/Transport.cs

Lines 200 to 207 in a9d280c

	public virtual void OnApplicationQuit()
	{
	// stop transport (e.g. to shut down threads)
	// (when pressing Stop in the Editor, Unity keeps threads alive
	// until we press Start again. so if Transports use threads, we
	// really want them to end now and not after next start)
	Shutdown();
	}

[imerr]

Do we want to call OnDestroy twice?
Empty body would be better imo