cdn.mathjax.org is shutting down

see announcement at https://www.mathjax.org/cdn-shutting-down/

this will affect the language specification.

the needed change is actually in the scala/scala repo — we'll need to make the change on the 2.11.x branch and then merge it forward to 2.12.x and 2.13.x

(I don't think we archive old builds of the spec, so I don't think there's anything to go back and update there)