Cabal (the library) advises use of `extensions:` when that field is deprecated

[mpilgrem]

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Motivation: https://github.com/commercialhaskell/stack/issues/6589

If a Cabal file uses ghc-options: to set language extensions, Cabal (the library) will advise the use of extensions: even when extensions: has been deprecated in favour of default-extensions:.

EDIT: This appears to be true even if the cabal-version: specifies a Cabal package format version that no longer supports extensions: at all.

[ulysses4ever]

That's an oversight. PR fixing it is welcome!

[mpilgrem]

I'll try to give the fix some attention in the coming days.