ox-html.el: add lang label to authinfo src blocks

* lisp/ox-html.el (org-html-style-default): `authinfo-mode' is defined
in Emacs 27. As such, in the CSS add an "Authinfo" lang label to
authinfo src blocks.

lisp/ox-html.el

@@ -308,6 +308,7 @@ property on the headline itself.")
   /* Languages per Org manual */
   pre.src-asymptote:before { content: 'Asymptote'; }
   pre.src-awk:before { content: 'Awk'; }
+  pre.src-authinfo::before { content: 'Authinfo'; }
   pre.src-C:before { content: 'C'; }
   /* pre.src-C++ doesn't work in CSS */
   pre.src-clojure:before { content: 'Clojure'; }