Update MariaDB maximum supported version to 10.8

MariaDB 10.8 is a short-term maintenance stable (GA) series of MariaDB installed by default with repo setup:

• https://mariadb.com/docs/reference/mariadb_repo_setup/
• https://mariadb.com/kb/en/changes-improvements-in-mariadb-108/
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  Update MariaDB maximum to 10.8.