Test old metadata versions

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. https://github.com/haskell/ghcup-metadata/issues/163

Prior work: https://github.com/haskell/ghcup-hs/pull/865

So: we probably want proper yaml schemas.

Start with the linked Version.hs reference and prior ghcup-hs PR #865, then inspect the metadata files and the compatibility problem described in issue #163. Define the scope of versioned YAML schemas and determine how compatibility would be tested with older ghcup binaries; done means the intended schemas and validation approach are documented or implemented.

Written by the indexing model from the issue text.