[TASK] Update core-testing-php84 image version in runTests.sh

This change raises the core-testing-php84
image version to use PHP 8.4.0 alpha2.

[1] https://git.typo3.org/typo3/CI/testing-infrastructure/-/commit/abd0b88dd70d0053359b08584d66df17540f177d

Resolves: #104480
Releases: main, 12.4, 11.5
Change-Id: Ife69d5fdb82f4243547fab5998b4a1ba0123bafc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85356


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>