Run tests against mariadb:10 instead of mariadb:latest

[mstenta]

MariaDB tests started failing recently with the following error:

SQLSTATE[HY000] [2054] Server sent charset (0) unknown to the client.

I traced this to the fact that we are running tests against the mariadb:latest Docker image, which is version 11.

Changing our docker-compose.testing.mariadb.yml configuration to use mariadb:10 fixes the issue.

I didn't dig into the root cause of this issue in MariaDB 11, but this PR will get our tests running again as a first step. Worth noting: we also pin our PostgreSQL tests (postgres:13), so this is consistent with that.

We may want to open a separate follow-up issue to explore why MariaDB 11 doesn't work, and see if we need to document this in https://farmos.org/hosting/install/#database-server...