[TASK] Upgrade used PHP 8.4 image

This change upgrades the version of the used
`typo3/core-testing-php84` image [1], which
uses PHP8.4 beta3 now along a new version of
composer (2.7.8).

[1] https://git.typo3.org/typo3/CI/testing-infrastructure/-/commit/bd98c246f5f8e5e91ee835212122998ffa5a1298

Resolves: #104748
Releases: main, 12.4, 11.5
Change-Id: I6555332a05e5809e30381c3a6b363a84c0150815
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85794


Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>