Fabric server, 1.21

https://gist.github.com/BBoldt/1c8b0123f42538b5ac381564ffacf8db

There is always some oversight - thanks for the report!
I have fixed this now :)

The commit that fixed this together with a few additional things is cbeb3ff
However, please be aware that there are still LOTS of bugs on Fabric server environments.
I have reported this to Architectury API accordingly (since I think its their fault: architectury/architectury-api#523).
If it's not their fault, I will reopen this, though!

I have fixed this issue on Architectury API's end - they forgot to properly register the SpawnEntityPacket on the server side (see architectury/architectury-api#524).
However, I found two more small issues regarding 1.20.6 and 1.21 servers.
I have fixed them in the newest commit for both versions. Now, everything should be in order (hopefully)

P.S.: I have no idea when and if my fix will be published for Architectury API!