Start building Fedora 42, and drop Fedora 40

Fedora 40 EOL is coming up at 2025-05-15, following the recent release of Fedora 42.

Also makes some significant changes to the layering strategy for our Fedora base images, going from only layering container and build on the common bootstrap image, and building everything else from scratch, to layering most things, including build, on container (still on bootstrap)