Change MathJax CDN default since old one is shutting down.

New URL: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js
Announcement: https://www.mathjax.org/cdn-shutting-down/

NOTE:  The new URL requires a version number, which we'll have
to update manually in subsequent pandoc releases in order to
take advantage of mathjax improvements.

Closes #3544.

src/Text/Pandoc/App.hs

@@ -1303,7 +1303,7 @@ options =
     , Option "" ["mathjax"]
                  (OptArg
                   (\arg opt -> do
-                      let url' = fromMaybe "https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_CHTML-full" arg
+                      let url' = fromMaybe "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS_CHTML-full" arg
                       return opt { optHTMLMathMethod = MathJax url'})
                   "URL")
                  "" -- "Use MathJax for HTML math"