Epic #72333: Clean up charset conversion and language service
Remove iconv usage and use mbstring polyfill
Iconv has subtle different behaviour than mbstring, hence unit tests fail with iconv.
Currently the testing framework does not test iconv (and the fallback) explicitly, hence we do not want to put effort into further supporting this in the future.
On top of that we already have the mbstring-polyfill in place through symfony. We should leverage this and therefore reduce our code size.