Setup dependencies are in 1.23, not 1.21.

Addresses https://github.com/haskell/cabal/pull/2515#commitcomment-10654094.

Cabal/Distribution/PackageDescription/Check.hs

@@ -1062,11 +1062,11 @@ checkCabalVersion pkg =
         ++ "compatibility with earlier Cabal versions then you may be able to "
         ++ "use an equivalent compiler-specific flag."
 
-  , check (specVersion pkg >= Version [1,21] []
+  , check (specVersion pkg >= Version [1,23] []
            && isNothing (setupBuildInfo pkg)
            && buildType pkg == Just Custom) $
       PackageBuildWarning $
-           "Packages using 'cabal-version: >= 1.22' with 'build-type: Custom' "
+           "Packages using 'cabal-version: >= 1.23' with 'build-type: Custom' "
         ++ "must use a 'custom-setup' section with a 'setup-depends' field "
         ++ "that specifies the dependencies of the Setup.hs script itself. "
         ++ "The 'setup-depends' field uses the same syntax as 'build-depends', "
@@ -1446,7 +1446,7 @@ checkDevelopmentOnlyFlags pkg =
                          -> CondTree v c a
                          -> [([Condition v], b)]
     collectCondTreePaths mapData = go []
-      where 
+      where
         go conditions condNode =
             -- the data at this level in the tree:
             (reverse conditions, mapData (condTreeData condNode))