[TASK] Raise php-cs-fixer dev dependency to 3.15

PHP-CS-Fixer 3.15 is now officially PHP 8.2
compatible, so we raise this dependency.

See https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/releases/tag/v3.15.0

Used command:
  composer req --dev friendsofphp/php-cs-fixer:^3.15

Resolves: #100226
Releases: main, 11.5
Change-Id: I21041eae62d32aafba3b7c33edd6e2b975ada957
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78180


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>