NeoForge installer fails with "expected at least three parts" when trying 1.21

[Reported in Discord](https://discord.com/channels/660567679458869252/660569641550217327/1255926359105081460)

![image](https://github.com/itzg/docker-minecraft-server/assets/988985/74c58df2-17e8-4205-ba9e-338ca6f6fd39)
