Support for GHC versions that do not have Stackage snapshots

[noughtmare]

I would like to summon GHC 9.6.2 projects, but that is not possible yet. So, I've quickly looked into the source code and what kind of changes that would require. Adding the metadata seems simple for the most part, except that each GHC version seems to need a latestLts Stackage snapshot associated with it.

I could change latestLts to return a Maybe Text rather than just Text to represent missing Stackage snapshots. But that raises the question what should happen when Stackage versions are missing. I guess the script command could simply show an error message, but the problem of summoning a project with multiple GHC versions of which one is not supported seems more difficult to solve. We could silently leave that version out, show a warning, or refuse to summon the whole project with an error.

What would be the best way to resolve this?