Our System check currently marks some PHP extensions as required that we actually don't need to require.
Those two are meanwhile built in PHP and can't be unavailable, so doesn't make sense to check them at all.
We indirectly already had
symphony/polyfill-mbstring included in the composer requirements, so the methods are always available. Checking for the extension is therefor not needed anymore, as everything should work without it. (We could also consider adding this as recommended module)
symphony/polyfill-iconv to ensure those methods are always available. We are actually not using that often, but the mbstring polyfill uses it and it allows us to remove the requirement. (We could also consider adding this as recommended module)
In addition I have marked or
Common::mb_* methods as deprecated and replaced their usage with the
mb_* functions directly, as those methods are actually doing nothing else. (We should do that for our plugins later as well)
btw just in case it was planned be good to not replace any further upgrade methods since we don't have any issues there currently and it doesn't really improve anything.