Upgrade buse to Stretch

Originally created by @intrigeri on #13528 (Redmine)

Stretch has Ruby 2.3, that’s supported in Redmine 3.3 and newer, so tails/sysadmin#11808 must be done first.

The only other potential problem I can foresee is the MySQL → MariaDB migration, but this went fine on ecours.

Parent Task: tails/sysadmin#12160

Edited May 15, 2020 by intrigeri