Remove another usage of mysql_get_client_version()

As of MariaDB 10.6, mysql_get_client_version returns the C library version, which breaks the version comparison.

This is a partial backport of 8ab3b54f, the Qt 6 commit that (among other things) removes this usage. Backporting the full commit seems overkill, we backport only the mysql_get_client_version() removal which should be safe according to upstream devs in https://bugreports.qt.io/browse/QTBUG-95071