I’m not sure if this is a temporary hiccup or not, but this issue resurfaced for me this evening. I was able to get things running by using the Rackspace mirror:
https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu
See 🔧 Change MariaDB PPA mirror to Rackspace by retlehs · Pull Request #1422 · roots/trellis · GitHub