chore(cabal): update haskell language edition to GHC2021

This includes a good set of default language extensions that are
often used by us. It frees us of explicitly importing common extensions.

Ref: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/control.html#extension-GHC2021

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

src/library/PostgREST/Config.hs

@@ -3,13 +3,10 @@ Module      : PostgREST.Config
 Description : Manages PostgREST configuration type and parser.
 
 -}
-{-# LANGUAGE FlexibleContexts      #-}
-{-# LANGUAGE FlexibleInstances     #-}
-{-# LANGUAGE LambdaCase            #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE RecordWildCards       #-}
+{-# LANGUAGE LambdaCase      #-}
+{-# LANGUAGE RecordWildCards #-}
 {-# OPTIONS_GHC -fno-warn-type-defaults #-}
-{-# LANGUAGE NamedFieldPuns        #-}
+{-# LANGUAGE NamedFieldPuns  #-}
 
 module PostgREST.Config
   ( AppConfig (..)