autopkgtest: Use mariadb-server-compat to have mysqld_safe (Closes: #1074780)

With the introduction of MariaDB 11.4, the mysqld_safe command has been replaced with mariadb-safe. The old command is however still available as a symlink provided by the mariadb-server-compat package, so switch to using it.

Upstream PowerDNS docs seem to be MySQL-only so we probably don't want to rewrite the backend to use properly MariaDB, but continue to use this 'compatibility layer' instead: https://doc.powerdns.com/authoritative/backends/generic-mysql.html