[keyserver] Switch Docker Compose from MySQL 5.7 to MariaDB 10.8

Summary: This switches our production config. I actually found it easier to start with prod on this one, but updates to the dev environment instructions and Nix config will follow.

Test Plan: I ran the web application and clicked around a bunch. Full test plan will come later

Reviewers: jon, atul, tomek

Reviewed By: atul, tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4499