noble: deb822 format support of /etc/apt/sources.list.d/ubuntu.sources instead of /etc/apt/sources.list

In ubuntu noble and later, cloud-init 24.1 based releases will automatically remove /etc/apt/sources.list and emit a warning log in cloud-init.log (and cloud-init status exit 2) if it finds in in images during first boot in favor of writing deb822 formatted /etc/apt/sources.list.d/ubuntu.sources files.

If livecd-rootfs is manipulating /etc/apt/sources.list in images which have cloud-init enabled, expect that this file will be removed by cloud-init on first boot and any tooling checking for cloud-init status will see a warning exit code.

The non-zero exit code (2) indicating a warning condition when removing /etc/apt/sources.list in cloud-init
is enough to break CI/CD tooling for scripts which rely on cloud-init status --wait exit codes.

Note: the behavior of cloud-init on mantic and earlier is to overwrite /etc/apt/sources.list on first boot which does This behavior of preferring unifed apt sources in /etc/apt/source.list will be retained on by cloud-init on earlier ubuntu releases even when 24.1 SRUs to stable releases

Also there is preinstalled image support in livecd-rootfs where cloud-init may not be enabled. It would be good to understand the implications if preinstall images migrate to /etc/apt/sources.list.d/ubuntu.sources instead of /etc/apt/sources.list. Be aware that images which contain duplicated sources defined /etc/apt/sources.list and /etc/apt/sources.list.d/ubuntu.sources would also generate warnings from apt like
LP: #2045086