Make haddock more robust to changes to the Language data type

With the introduction of GHC2021, the `Languages` data type in GHC will
grow. In preparation of that (and to avoid changing haddock with each
new language), this change makes the code handle extensions to that data
type gracefully.

(cherry picked from commit c341dd7)

Author: nomeata

haddock-api/src/Haddock/Backends/Xhtml.hs

@@ -223,10 +223,7 @@ moduleInfo iface =
           ("Language", lg)
           ] ++ extsForm
         where
-          lg inf = case hmi_language inf of
-            Nothing -> Nothing
-            Just Haskell98 -> Just "Haskell98"
-            Just Haskell2010 -> Just "Haskell2010"
+          lg inf = fmap show (hmi_language inf)
 
           multilineRow :: String -> [String] -> HtmlTable
           multilineRow title xs = (th ! [valign "top"]) << title <-> td << (toLines xs)