[TASK] Use correct mariadb default in `runTests.sh`

With #102518 the minimal suported version for MariaDB
has been raised, and the version selection `-i` for
functional tests adjusted.

Adjusting the default for MariaDB if no manual version
is selected has been missed, which is now corrected.

Resolves: #102808
Related: #102518
Releases: main
Change-Id: I12e3f64abad313c7d11ea0dbf7553bfbfc1a5f58
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82424


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>