[devel-noble] provide a mechanism for UT major release upgrade

Once we have a new set of system-image channels, we'll want to have a mechanism to automate channel switching, to encourage users to upgrade to a new major release.

What I have in mind for upgrading from 20.04 to (say) 25.3.x is to let lss-system-update read something similar to Ubuntu's meta release [1] but in JSON format. Then have UI to do the channel switch.

We'll also have to think about how we represent daily/RC/stable to users.