[BUGFIX] Use mb_* methods directly instead of CharsetConverter

There are several places where mb_* methods are still not used, although
they are even better to use instead of the $csConv->parseCharset() method
for instance, as it has more aliases defined already.

Same goes for "entities_to_utf8" which is a PHP user-space implementation
of html_entity_decode() dated back to Kaspers' change in 2003.

Using native PHP methods should also increase performance marginally.

Resolves: #81575
Releases: master, 8.7
Change-Id: I5e97881cc7107883a5ff53d534161a7ec17e1ee5
Reviewed-on: https://review.typo3.org/53208


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>