[BUGFIX] Use mb_strlen in StringLengthValidator

By using mb_strlen for checking the length, the
check counts multibyte values now correctly.

Resolves: #79684
Releases: master
Change-Id: Idcc9fc6e77d0cd42bc43986acb956af67da2453d
Reviewed-on: https://review.typo3.org/51593


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>