jenkins: remove v16 from VersionSelectorScript
It's written down that we keep stuff around for 6 months after a release line goes End-of-Life before removing.
https://github.com/nodejs/build/blob/2e7c763cc6b02eee6edbe1f5199a54350b273e2a/README.md?plain=1#L294-L296
FWIW I can only remember one time when we decided to do an additional release after End-of-Life, which IIRC was within the month after the End-of-Life.
I would be good to keep this around a bit longer if possible. I don't think it hurts anything, right?
That's why I made this a draft. I'll come back to it in 6 months.
Ready for a new review.