[!!!][TASK] Require PHP mbstring and intl extensions

* mbstring: This is one of the most common extensions,
  often compiled directly into default packages, or
  available as additional package. While the symfony
  polyfill package adds the most common functions, it
  is not complete and we're already working around
  possible non-existance in a couple of places. The
  native functions are quicker and it should be
  fair to require mbstring in a modern PHP 8.1 build.

* intl: We really need this package to finally solve
  various sorting and date issues in the system. The
  symfony polyfills are partially stubs only that
  implement only a small set of features for
  english-only. This extension is the only way to
  finally drop dependencies to the nasty locales,
  which is a far bigger burden on systems than
  requiring intl.

$ composer req ext-mbstring
$ composer req ext-mbstring -d typo3/sysext/core --no-update
$ composer req ext-intl
$ composer req ext-intl -d typo3/sysext/core --no-update
$ composer rem symfony/polyfill-mbstring
$ composer rem symfony/polyfill-mbstring -d typo3/sysext/core --no-update
$ composer rem symfony/polyfill-mbstring -d typo3/sysext/frontend --no-update
$ composer rem symfony/polyfill-intl-icu
$ composer rem symfony/polyfill-intl-icu -d typo3/sysext/core --no-update
$ composer rem symfony/polyfill-intl-idn
$ composer rem symfony/polyfill-intl-idn -d typo3/sysext/core --no-update
$ composer rem symfony/polyfill-intl-normalizer
$ composer rem symfony/polyfill-intl-normalizer -d typo3/sysext/core --no-update

Change-Id: I91bbd65862ceef7504563df71959e705666fb74a
Resolves: #96889
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73492


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>