Sphinx 9 introduced MathJax 4 as default, with the corresponding mathjax4_config option. This does not work with the MyST mathjax_ignore CSS classes, since the mathjax config override does not now about mathjax4_config. This PR makes it work also for MathJax 4. Note that the configuration for MathJax 4 and 3 is identical, so it can be handled in the same block.