Last update generated an error: filesystem mounted at /boot has no free disk space! (Solution)

OS type and version: Debian, Debian-based (e.G. Ubuntu)
Virtualmin/Webmin version: any

Hi there,
This is actually not a question, but an answer:
If you find the above error message after a running a packages upgrade then

sudo apt autoremove 

from the commandline. This will remove the outdated files from /boot.
Sometimes apt won’t run because there is not enough space left in /boot.
In that case follow this guide to safely clean up the /boot folder:
kernel - What is the safest way to clean up /boot partition? - Ask Ubuntu

Note: I am posting this because I had almost followed a bad advice given in another thread of this Forum. That thread is closed and I can not answer there, which is not cool!