cabal init -i: add the GHC2021 language option

[ulysses4ever]

fix #8265

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[jneira]

good work, I would say this is in the limit of needing tests, maybe an unit test?

[ulysses4ever]

@jneira there's a test that the new language is on the list:
https://github.com/haskell/cabal/pull/8277/files#diff-087fb21b3293d2ef32df883500fad0ead4fa4281b0ba0d376ea36928164567afR915

What else can be tested here?

[jneira]

hmm, maybe not an unit test but a test checking the produced cabal file has the correct language set, but I hope it is already tested with other values somewhere

[jneira]

lgtm, thanks again!

[ulysses4ever]

@jneira thanks! I went ahead and added a test that actually checks the contents of the cabal file.

[ulysses4ever]

@mergify backport 3.8

[mergify]

backport 3.8

✅ Backports have been created

• #8280 cabal init -i: add the GHC2021 language option (backport #8277) has been created for branch 3.8