use puppet/systemd 6.0 features

[evgeni]

In #1883 we deployed redmine with Puma using systemd.

As our infra currently uses puppet/systemd 4.2.0, we could not use nice features that were introduced in newer versions of that module and had to resort to write the systemd unit files ourself.

Once the rest of the infra has been updated to support puppet/systemd 6.0 (or newer), we should revert the "make code compatible with systemd 4.2.0" commit, thus switching to the more modern style of managing things.