Retire DNF builder, transition to bootc builder

[korewaChino]

See title.

Let's retire the DNF root provisioner and provision the filesystem using OCI images instead, by doing something like this:

builder: bootc

bootc:
    image: ghcr.io/ultramarine-linux/plasma-bootc:latest-standard

The -standard images are similar to the normal bootc images with the OSTree-specific stuff removed, allowing them to be used as a plain filesystem for a standard mutable installation with minimal changes by simply copying over the files using either rsync (like Anaconda) or with systemd-repart.

[linear]

FYRA-539