Maintain all Fedora versions in base-images `main` branch

We're currently looking at inheriting from tier-x in FCOS as a git submodule. But one challenge with that is that base-images uses branching for Fedora versions. This makes it incompatible with how the fedora-coreos-config uses branches and copies configs between them.

The proposal is to have the main branch be used for all the Fedora versions, and use treefile conditionals if changes are truly needed across major versions. This matches how e.g. dist-git branches are often maintained, with the rawhide branch containing the canonical spec file with any necessary conditionals so that it can just be git merged into the other release branches.