Enable GHC2021 language extension set.

This sadly leads to a GHC bug.

    building
    Preprocessing library for deriving-trans-0.6.1.0..
    Building library for deriving-trans-0.6.1.0..
    [1 of 7] Compiling Control.Monad.Accum.OrphanInstances ( src/Control/Monad/Accum/OrphanInstances.hs, dist/build/Control/Monad/Accum/OrphanInstances.o, dist/build/Control/Monad/Accum/OrphanInstances.dyn_o )
    [2 of 7] Compiling Control.Monad.Select.OrphanInstances ( src/Control/Monad/Select/OrphanInstances.hs, dist/build/Control/Monad/Select/OrphanInstances.o, dist/build/Control/Monad/Select/OrphanInstances.dyn_o )
    [3 of 7] Compiling Control.Monad.Trans.Elevator ( src/Control/Monad/Trans/Elevator.hs, dist/build/Control/Monad/Trans/Elevator.o, dist/build/Control/Monad/Trans/Elevator.dyn_o )
    [4 of 7] Compiling Control.Monad.Trans.Compose.Transparent ( src/Control/Monad/Trans/Compose/Transparent.hs, dist/build/Control/Monad/Trans/Compose/Transparent.o, dist/build/Control/Monad/Trans/Compose/Transparent.dyn_o )
    [5 of 7] Compiling Control.Monad.Trans.Compose ( src/Control/Monad/Trans/Compose.hs, dist/build/Control/Monad/Trans/Compose.o, dist/build/Control/Monad/Trans/Compose.dyn_o )
    src/Control/Monad/Trans/Compose.hs:189:37: error:ghc: panic! (the 'impossible' happened)
      (GHC version 9.2.4:
            No skolem info:
      [k_aeaQ]
      Call stack:
          CallStack (from HasCallStack):
            callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc:GHC.Utils.Panic
            pprPanic, called at compiler/GHC/Tc/Errors.hs:2912:17 in ghc:GHC.Tc.Errors
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

To reproduce this bug, just run `nix build`.

deriving-trans.cabal

@@ -63,7 +63,7 @@ library
     Control.Monad.Trans.Compose.Transparent
     Control.Monad.Trans.Elevator
   build-depends:
-    , base < 5
+    , base >= 4.15.0.0 && < 5
     , monad-control
     , monad-control-identity
     , transformers >= 0.6
@@ -90,33 +90,14 @@ library
     build-depends:
       , unliftio-core
   hs-source-dirs: src
-  default-language: Haskell2010
+  default-language: GHC2021
   default-extensions:
-    BangPatterns
-    ConstraintKinds
     DataKinds
-    DeriveFoldable
-    DeriveFunctor
-    DeriveGeneric
-    DeriveTraversable
     DerivingStrategies
     DerivingVia
-    EmptyDataDeriving
-    FlexibleContexts
-    FlexibleInstances
     FunctionalDependencies
-    GeneralizedNewtypeDeriving
-    ImportQualifiedPost
-    MultiParamTypeClasses
-    NamedFieldPuns
     OverloadedStrings
-    RankNTypes
-    StandaloneDeriving
-    StandaloneKindSignatures
-    TupleSections
-    TypeApplications
     TypeFamilies
-    TypeOperators
   ghc-options:
     -Weverything
     -Wno-implicit-prelude