Use systemd.timer instead of a cron job

The rational is that we need to spread the load on the mirrors
that apt update and unattended-upgrades cause. To do so, we
leverage the RandomizeDelay feature of systemd. The other advantage
is that the timer is not run at a fixed daily.daily time but
instead every 24h. This also fixes the problem that the randomized
deplay in the current apt.cron.daily causes other cron jobs to
be deplayed.

A compatibility cron job is also provided for systems that do not
use systemd.

Note that the time is fired two times a day, but the logic inside
of apt.systemd.daily will ensure (via stamp files) that the
servers are hit at most every 24h. Firing two times a day helps
with the worst case update time and it also helps with systems
that are not always on.

LP: #246381, #727685
Closes: #600262, #709675, #663290

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