Update polyfill to non-malware containing variant

Aparently polyfill was bought by some Chinese company and has been used to push malware to mobile devices.
While polyfill is not required anymore by mobile browsers, just to be safe the old link will be updated to a cdn owned by CloudFlare.

layouts/shortcodes/mathjax.html

@@ -17,7 +17,7 @@
     });
 
 </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 type="text/javascript" id="MathJax-script" async
   src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
 <style>