Build with GHC 9.12

[andreasabel]

Blocked on:

• HsOpenSSL because of Custom setup: Can't use Cabal-3.14 released with GHC 9.12 haskell/cabal#10467
• GHC 9.12 fizruk/http-api-data#147
• https://codeberg.org/valpackett/http-link-header/issues/1

See:

• Bump CI to GHC 9.12 #514