krtpowa
The version of DietPi has no real relation to the Debian version. There will be no new version for MariaDB as you are on Stretch. You would need to upgrade to Debian Buster to get further updates. Current version of MariaDB on Buster is 10.3.
Even if we did not support Debian version upgrades, you might have a look into this post https://dietpi.com/forum/t/qbittorrent-version-shipping-with-dietpi/2326/9