Test old metadata versions

[hasufell]

The metadata is not properly versioned, except for the filenames that correspond to the hardcoded URL in the ghcup binary: https://github.com/haskell/ghcup-hs/blob/55030d83da6d59196df28c0eb115cdf19ff20393/lib/GHCup/Version.hs#L37

To test compatibility you currently would have to download an older ghcup binary and test it with e.g. ghcup-0.0.7.yaml to truly know.

That lead to e.g. #163

Prior work: haskell/ghcup-hs#865

So: we probably want proper yaml schemas.

[fendor]

That's a good idea, I was thinking of adding yaml schemas for the hie.yaml file format as well, as it is a little bit too ad-hoc and would benefit from a schema definition. Additionally, it can help IDEs to provide auto completions.