Allow `NURSERY` in JSON Schema (#7374)
## Summary
At some point, we removed these so that they wouldn't be autocompleted
for users, since we wanted to discourage usage of `ALL`. But given that
they're valid values, I think that was a bad idea -- it leads to an even
more confusing experience whereby JSON Schema validators tell you that
you have an error, when you don't.
Closes https://github.com/astral-sh/ruff/issues/7261.
