Add “Add default-language” suggestion (backport #9766)

[mergify]

Closes #9620

Add a warning ("Hackage would refuse this package") when default-language is missing. Done to prepare the user to GHC language editions.

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• Patches conform to the coding conventions.
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• Manual QA notes have been included.
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** QA notes **

1. cabal init a package, choose cabal-version 3.4
2. run cabal check
3. old cabal: it will not complain about missing default-language
4. new cabal: it wil complain about missing default-language

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   This is an automatic backport of pull request Add “Add default-language” suggestion #9766 done by Mergify.

[ulysses4ever]

This technically doesn't qualify for bugfix, perhaps, but having consistent behaviour of cabal check between master and 3.12 is important.