Inherit from fedora-bootc's tier-x on Fedora 42+

[jlebon]

There is a new tier-x in the fedora-bootc project whose goal is to provide a common base that all variants (including tier-1) can share.

Move FCOS over to use this new tier, but only starting from Fedora 42.

This is a profound change and the start of an exciting new future! This formalizes our relationship to other image-mode variants, encouraging us to innovate and solve problems together in a more direct way.

Put more practically, e.g. bug fixes, new features, or temporary workarounds that concern all/most tier-x derivatives should probably be carried out at the tier-x level rather than the CoreOS level.

Eventually, this inheritance will be made even more explicit by having FCOS be built FROM the tier-x image. For now, we share at the manifest level, which is a stepping stone towards that goal.

Patches to actually dedupe our manifests with tier-x will follow. Though note there is no change in the resulting package set here.