systemctl: rename enqueue-marked-jobs to enqueue-marked (#40930)

Closes #40883. As described in the issue, it's not "jobs" that are
marked, and also the name is unnecessarilly long.

I think we don't need any compatibility measures here. At least in the
rpm world, package upgrade scripts go through the helper which is part
of the package so the new systemctl and the new helper are upgraded
together.