Upgrade puppet-git.lizard to Stretch

Originally created by @intrigeri on #13529 (Redmine)

This should be pretty much straightforward as long as we stick to our current Puppet packages, and Gitolite keeps working.

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

Parent Task: tails/sysadmin#12160

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