Silently mark packages requiring an unsupported version of opam as unavailable

[kit-ty-kate]

Fixes https://github.com/ocaml/opam/issues/5631

This makes it possible to have a smoother upgrade path for the opam repository when opam adds a new syntax for example.

In the future it would be nice to have a specific error message to tell the user why this package is not available, but I feel like we can do this later (TODO: open a ticket for it if this is merged first)

[rjbou]

Updated the PR

[kit-ty-kate]

rebased