fix: circleci pin to stable debian distro & its mariadb

Just look at #190 latest comments

Prior this was pinned to sid, which is debian experimental and does not enjoy the stability.
It looks like there was a narrow time window when mariadb-client-10.6 was available

Decided to install bullseye mariadb-client, which is 10.5 so I rolled back mariadb (it's likely WordPress isn't using advanced enough functionality to care)

worth a note that more work is needed anyway: circleci warns image is deprecated as they now have circleci 2.0 images lol 😆

wp-cli / scaffold-package-command

fix: circleci pin to stable debian distro & its mariadb #224