core: migrate to platformdirs

[karlicoss]

https://github.com/ActiveState/appdirs is deprecated, and they are recommending switch to platformdirs
I think it's fine to do it without a fallback, according to https://robert-96.github.io/top-pypi-packages/, it's one of the most popular packages, so very likely already installed. Also it's only used for cachew, so even if it breaks, not a huge issue.