refactor: replace `polyfill.io` for `cdnjs` hosted link (#1598)

_data/origin/cors.yml

@@ -8,8 +8,8 @@ cdns:
   - url: https://fonts.googleapis.com
   # jsDelivr CDN
   - url: https://cdn.jsdelivr.net
-  # polyfill.io for math
-  - url: https://polyfill.io
+  # polyfill.io for math (cdnjs.cloudflare.com/polyfill)
+  - url: https://cdnjs.cloudflare.com
 
 # fonts
 

_includes/js-selector.html

@@ -91,7 +91,7 @@
       }
     };
   </script>
-  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
+  <script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>
   <script id="MathJax-script" async src="{{ site.data.origin[type].mathjax.js | relative_url }}"></script>
 {% endif %}