upgrades: Unattended upgrades must run at midnight (00:00) local time

Unattended upgrades are expected to run at midnight. But they seem to be running at around 6 am in the morning (+/- 30 minutes).

First reported on the forums: https://discuss.freedombox.org/t/change-auto-update-time/550

Configure unattended-upgrades to run at midnight. Changes should apply to all users, including existing installations.