I just noticed that I haven’t logged on to one of my RaspberryPIs for quite a while and also didn’t update it. Instead of hacking some root crontab, a quick search brought this thread in the Raspi forums up: crontab update – Raspberry Pi Forums and unattended upgrades – How do I enable automatic updates? – Ask Ubuntu
To keep the Raspberry Pi updated, I just use the following:
sudo apt install unattended-upgrades apt-listchanges
sudo dpkg-reconfigure -plow unattended-upgrades
Documentation can be found here: Package management | Ubuntu