systemd unit: update to systemd v253
WARNING: I just could not get it to compile with latest main branch.
No Qt6, KF6, ... here yet.
I did git checkout tags/v5.103.0
as that seems to be the latest published version.
Compilation worked fine after that, but I ended up with a detached git head.
No idea about a better approach.
Hope it still works for merging.
From systemd side there are mostly the usual additions of options and option values.