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