adopting @haskell-lang-sig

[Jens-Ulrik Petersen]

Hi,

We have discussed this in the past and the latest unwanted
package-retirement is now the last straw for me, so I want to go ahead with
adding @haskell-lang-sig group to all Haskell ghc-* library packages at
least.

See https://pagure.io/fedora-infrastructure/issue/10891
and https://pagure.io/fesco/issue/3143

Happy New Year,
Jens