You will find after your upgrade that your NetworkManager.cs script claims that Unity claims that you can no longer override methods OnServerConnect, OnServerDisconnect, and OnServerAddPlayer – this is actually not true, but the signature for these methods has changed. They now require an object of type NetworkConnectionToClient as their argument (instead of NetworkConnection) – so your override method lines should look similar to this now:
public override void OnServerAddPlayer(NetworkConnectionToClient conn)
{
// do stuff
}
*If you are interested in how I came to that solution, I used the inspector in my editor to dive into Mirror’s code and looked the the original parent NetworkManager.cs (in Mirror/Runtime/NetworkManager.cs) and I saw the new method signatures there. Note that this whole method is obsolete, but it works still 
*
For an official solution on how to fix this – check this other thread NetworkManager.OnClientConnect is obsolete
Also, as a side note: As of April 2022, Telepathy is no longer deprecated. It (and KCP) are both supported as of Mirror 66.0.9. I can only assume that vis2k (the author of Mirror) deprecated and then revived it since he uses it for his MMORPG assets.