Bump default Mathjax version up to v3 #462

Currently Kaleido loads MathJax v2.7.5 by default: https://github.com/plotly/Kaleido/blob/8e1d783aa9478adc7fc3d940515d3e847e7aee45/src/py/kaleido/_page_generator.py#L21

That's a pretty old version from 2018, so we should probably bump up to 3.2.2 by default.

CDN link: https://cdn.jsdelivr.net/npm/mathjax@3.2.2/es5/tex-svg.js

This is also a blocker for dropping MathJax v2 support in Plotly.js. We need to release a Kaleido version which defaults to MathJax v3 before making a Plotly.py release which drops MathJax v2 support.