Reconsider warning about missing languages

> It's a curious problem. The GHC* "languages" will multiply in the future (iiuc GHC SC's current intentions). So, simply two branches (if/else) won't be enough soon. I'm wondering if we should support something like `default-language: GHC` which would mean the latest supported GHC*.
>
> Semantically, saying "give me the latest GHC language that this compiler supports" comes dangerously close to not specifying a language — the very situation `cabal check` was supposed to flag. But I think this idea behind the check hasn't aged well (due to invention of GHC* "languages") and we should revisit it.

_Originally posted by @ulysses4ever in [#11017](https://github.com/haskell/cabal/issues/11017#issuecomment-3004574096)_.

Consider also what @tonyday567 is doing in #11017, abstracting away/backporting language extensions so they are usable to older GHCs. It is a valid usecase.