Change default revealjs-url to use CDN version of revealjs v4.

See #6408.

MANUAL.txt

@@ -2146,7 +2146,8 @@ All [reveal.js configuration options] are available as variables.
 To turn off boolean flags that default to true in reveal.js, use `0`.
 
 `revealjs-url`
-:   base URL for reveal.js documents (defaults to `reveal.js`)
+:   base URL for reveal.js documents (defaults to
+    `https://unpkg.com/reveal.js@^4/`)
 
 `s5-url`
 :   base URL for S5 documents (defaults to `s5/default`)

src/Text/Pandoc/Writers/HTML.hs

@@ -340,7 +340,7 @@ pandocToHtml opts (Pandoc meta blocks) = do
                   defField "slidy-url"
                     ("https://www.w3.org/Talks/Tools/Slidy2" :: Text) $
                   defField "slideous-url" ("slideous" :: Text) $
-                  defField "revealjs-url" ("reveal.js" :: Text) $
+                  defField "revealjs-url" ("https://unpkg.com/reveal.js@^4/" :: Text) $
                   defField "s5-url" ("s5/default" :: Text) $
                   defField "html5" (stHtml5 st)
                   metadata