Add debian backports repo

The

# add the backports repo to the source.list.d dir
echo "deb http://deb.debian.org/debian bullseye-backports main" | sudo tee /etc/apt/sources.list.d/bullseye-backports.list

# update the repo index
sudo apt update

# install a package by using the backports repo
sudo apt -t bullseye-backports install openssl