Add codename `Hydrogen` for node 18 to `README.md` and `schedule.json` file

[HonkingGoose]

The CODENAMES.md file says that version 18.x has the Codename Hydrogen: https://github.com/nodejs/Release/blob/0f3230611ebcae7488e55ad1fffdf989a787a983/CODENAMES.md?plain=1#L11

But these files don't list the Hydrogen codename for to the 18.x release:

https://github.com/nodejs/Release/blob/0f3230611ebcae7488e55ad1fffdf989a787a983/README.md?plain=1#L9

https://github.com/nodejs/Release/blob/0f3230611ebcae7488e55ad1fffdf989a787a983/schedule.json#L98-L104

[HonkingGoose]

Oh, I see Node 18.x is not yet released as stable LTS. Is that's why it doesn't have a codename in all the files?

[richardlau]

Oh, I see Node 18.x is not yet released as stable LTS. Is that's why it doesn't have a codename in all the files?

Correct. The codename only applies once the release has become LTS.

[HonkingGoose]

Fixed by commit: https://github.com/nodejs/Release/commit/7277c242ca98cf3eb12b4e0c479ec5a5834f02a0