This is a follow-up to T271876: Replace 'refresh' process with Symfony caching

Currently, we have a /refresh endpoint that is used to cache MediaWiki:Epub.css. This custom caching can now be replaced by standard Symfony caching, which will mean less code and that this data will be able to be cleared along with the rest of the cache with ./bin/console c:p:c app.cache.

The CSS should be cached for 1 month.

The /refresh endpoint should be updated to delete the relevant cache item (after which it'll be updated when next required by an export).