Move to composefs native backend with bootc

[travier]

Describe the enhancement

Once the composefs native backend is ready in bootc, we should move existing systems to use it.

The composefs native backend is only expected to be implemented in bootc (and not rpm-ostree) so moving to this backend requires completing the transition to bootc, which is tracked in #1726. The current main blockers are:

• Support for bootc in Zincati
• Some form of support for package layering in bootc

For #1719, we will only use the composefs backend thus those images will start from the beginning with that support only.

This is tracked upstream in bootc in bootc-dev/bootc#1190

To make it possible to migrate existing systems in place, we should limit changes as much as possible. It should be possible to do this change without any disk layout changes nor bootloader configuration changes.

System details

All systems

Additional information

No response