[BUGFIX] Bump symfony/polyfill-mbstring to ^1.16

ext:frontend uses the function mb_ord which is available since
symfony/polyfill-mbstring v1.16 and to avoid issue this dependency is
properly increased and also added to ext:frontend.

Composer commands used:
* composer require "symfony/polyfill-mbstring:^1.16"
* composer require --no-update -d typo3/sysext/core "symfony/polyfill-mbstring:^1.16"
* composer require --no-update -d typo3/sysext/frontend "symfony/polyfill-mbstring:^1.16"

Resolves: #95279
Releases: master, 10.4, 9.5
Change-Id: I31aa9affa936ae96d5a02ad846aa011fb89af75d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71127


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>