[keyserver] Use mysqldump from MariaDB 10.8 instead of MySQL 8

Summary:
In D4183 we decided to use mysqldump from MySQL 8 instead of MySQL 5.7 because it was too challenging to get the right one.

Now that we're using MariaDB 10.8, it's easier to get the right mysqldump for the given database server.

Test Plan: I tested a backup from this updated image and then tested a restore of that backup on the same image

Reviewers: atul, tomek

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4641