appstream: Handle pre-release distro-upgrade kind explicitly

AppStream 1.0.0 introduces AppStream::Release::KindSnapshot
https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Releases.html

We want to only allow KindStable or optionally KindDevelopment,
and disallow KindSnapshot (such as Fedora Rawhide)

If there is a will to enable upgrading to such releases, it should
be under a separate user toggle.