packer: add Ubuntu 22.04 image

The installation is now using cloud-init config instead of preseed files
(see https://ubuntu.com/server/docs/install/autoinstall).

For now, it's not possible to have different file names than "user-data"
and "meta-data" and the hostname is hardcoded in "user-data" so it won't
work for Ubuntu 24.04 (see
https://cloudinit.readthedocs.io/en/latest/topics/datasources/nocloud.html).