[#6342] bump markdown version, invalidate markdown cache, for small fix.
Would be good to upgrade much further, but there are various regressions,
mostly with markdown internal placeholders showing up in the output, which is
no good. This starts at 2.3 and perhaps is because of our own extensions
needing update, but also I think due to handling invalid markdown (e.g.
markdown within html) differently -- but I don't want that to break. More info
at https://github.com/Python-Markdown/markdown/issues/458 and