Bump haddock version to 2.31.0

haddock-api/haddock-api.cabal

 cabal-version:        3.0
 name:                 haddock-api
-version:              2.29.0
+version:              2.31.0
 synopsis:             A documentation-generation tool for Haskell libraries
 description:          Haddock is a documentation-generation tool for Haskell
                       libraries

haddock.cabal

 cabal-version:        3.0
 name:                 haddock
-version:              2.29.0
+version:              2.31.0
 synopsis:             A documentation-generation tool for Haskell libraries
 description:
   This is Haddock, a tool for automatically generating documentation
@@ -156,7 +156,7 @@ 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 == 2.29.0
+    build-depends:  haddock-api == 2.31.0
 
 test-suite html-test
   type:             exitcode-stdio-1.0