Revert "Bump haddock and haddock-api to 2.20.0"

This reverts commit 81ff5e9e.

haddock-api/haddock-api.cabal

 cabal-version:        2.0
 name:                 haddock-api
-version:              2.20.0
+version:              2.19.1
 synopsis:             A documentation-generation tool for Haskell libraries
 description:          Haddock is a documentation-generation tool for Haskell
                       libraries

haddock.cabal

 cabal-version:        2.0
 name:                 haddock
-version:              2.20.0
+version:              2.19.0.1
 synopsis:             A documentation-generation tool for Haskell libraries
 description:
   This is Haddock, a tool for automatically generating documentation
@@ -153,7 +153,7 @@ executable haddock
   else
     -- in order for haddock's advertised version number to have proper meaning,
     -- we pin down to a single haddock-api version.
-    build-depends:  haddock-api == 2.20.0
+    build-depends:  haddock-api == 2.19.0.1
 
     other-modules:
       ResponseFile