forcer les sources apt

cat << EOF > /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu/ focal main restricted
deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted
deb http://archive.ubuntu.com/ubuntu/ focal universe
deb http://archive.ubuntu.com/ubuntu/ focal-updates universe
deb http://archive.ubuntu.com/ubuntu/ focal multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-updates multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ focal-security main restricted
deb http://security.ubuntu.com/ubuntu/ focal-security universe
deb http://security.ubuntu.com/ubuntu/ focal-security multiverse
EOF

installer

apt update && apt upgrade -y && apt-get --with-new-pkgs upgrade -y
do-release-upgrade -d

if asked, do not install grub :)

keep actual version (O) of /etc/sysctl
keep actual version (O) of /etc/ssh/ssh_config