Upgrade Debian 11 to 12

The upgrade of the underlying OS gets carried out manually (i.e., without the help of Terraform or Ansible).

Steps

Create a backup of the whole system, e.g., using the Hetzner cloud backup mechanism.
Change all apt repositories in /etc/apt/sources.list and /etc/apt/sources.list.d/* from bullseye to bookworm.
- non-free was changed to non-free-firmware.
$ apt update
$ apt full-upgrade
- If the systems asks to configure iperf3 as automatic service startup, select no.
- If the systems asks to restart services without manual interaction, select yes.
- If the systems asks to decide on how to handle updated config files in /etc/*, choose N (= keep your currently-installed version).
$ apt autoremove to clean up old packages.
Reboot.
(Obviously) Check if all services are up and running correctly.
Adapt the Terraform configuration, i.e., change the base image of the system from debian-11 to debian-12.
- Be careful when re-running Terraform because it usually wants to recreate the whole system. This can be ommitted by configuring the respective Terraform resource to ignore this specific attribute when considering rebuilding.
Adapt the Ansible confguration.
Adapt all necessary information in Ansible roles, e.g., it is necessary to update bullseye to bookworm in apt repositories.