Update to MathJax 4

[MoshiKoi]

There is currently a MathJax issue raised in Subscript and superscript 3 displayed too low in MathJax.

We are currently on MathJax 3; Although it is a Safari/WebKit bug, MathJax 4 is supposed to have some improvements in Safari rendering (mathjax/MathJax#2866 (comment)), so we might want to update to that version.

[Oaphi]

For reference:

1. Upgrading from v3 to v4;
2. Breaking changes in v4;