[es-js] point to 16.x legacy docs

Author: spalger

conf.yaml

@@ -346,7 +346,7 @@ contents:
                 title:      JavaScript API
                 prefix:     javascript-api
                 current:    7.x
-                branches:   [ master, 7.x, 6.x, 5.x, 15.x ]
+                branches:   [ master, 7.x, 6.x, 5.x, 16.x ]
                 index:      docs/index.asciidoc
                 tags:       Clients/JavaScript
                 subject:    Clients