[TASK] Upgrade to phpunit ^10.5.21

The release comes with a nifty memory usage
improvement and probably allows us to avoid
a couple of hacks we've used, which we'll
clean up later. Raise for now. Core v12
will raise phpunit 10 instead.

> composer req --dev phpunit/phpunit:^10.5.21

Resolves: #104127
Releases: main, 12.4
Change-Id: I4faf1af0378e2996324b3db101709764a31419b0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84731


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>