system-test framework starts MariaDB server in the backend using both 'mysql' and 'mariadb' serices

e.g https://github.com/mariadb-corporation/MaxScale/blob/6.1/system-test/maxtest/src/mariadb_nodes.cc#L180

the problem initially caused by https://jira.mariadb.org/browse/MDEV-26612: if MariaDB start by 'service mysql start' failed and the we try 'start mariadb...' we can end up with two server processes running and screwed system.

suggestion: use only `mariadb` for all versions of backend except 10.2 and earlier. If we need support for mysql and Percona we need to check backend product.