[1.21.1 Alpha] Replace kubejs: namespace for kjs'd registry entries with gtceu:

[jurrejelle]

Right now when making registry entires, we put it in the kubejs; namespace instead of the gtceu: namespace
We want to revert that
See discussion here:

https://discord.com/channels/701354865217110096/1089296351906504835/1449754132633092157