refactor(templates): replace `polyfill.io` (#7032)

dev/templates/legacy.pug

@@ -52,7 +52,7 @@ html
 
     script(
       crossorigin='anonymous'
-      src='https://polyfill.io/v3/polyfill.min.js?features=EventSource'
+      src='https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=EventSource'
     )
 
     //- JS