Last update was {T176685} in 2017. The [currently used version](https://phabricator.wikimedia.org/diffusion/ECMI/browse/master/resources/lib/codemirror/lib/codemirror.js$9668) is 5.35.0. Changelog since then: https://codemirror.net/doc/releases.html. As far as I can see no incompatible changes have been made, but tons of bugfixes and some performance improvements.
Note there is a version 6 update planned as well (see T259059), but this will not happen soon.
To do:
[] Ping @MusikAnimal if this is ok.
[] Check if the copy in the extension does not have any modifications. We don't want to loose these without knowing.
[] Check the release notes again for changes that might turn out to be problematic.
[] Check what tests exist in the extension's codebase.
[] Replace the file in the extension.
[] Locally test if it still works.