fix: remove usage of polyfill.io

This closes a security concern detected by CodeQL

Signed-off-by: jmeridth <[email protected]>

Author: jmeridth

_includes/footer.html

@@ -11,7 +11,7 @@
 </footer>
 
 <!-- common js -->
-<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=es6&flags=gated"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?version=4.8.0" integrity="sha384-i0IGVuZBkKZqwXTD4CH4kcksIbFx7WKFMdxN8zUhLFHpLdELF0ym0jxa6UvLhW8/ sha384-3d4jRKquKl90C9aFG+eH4lPJmtbPHgACWHrp+VomFOxF8lzx2jxqeYkhpRg18UWC" crossorigin="anonymous"></script>
 <script src="{{ site.baseurl }}/assets/js/fetch.js"></script>
 <script src="{{ site.baseurl }}/assets/js/collections.js"></script>
 <script src="{{ site.baseurl }}/assets/js/dom.js"></script>
@@ -30,4 +30,3 @@
 
   gtag('config', 'UA-1265430-2');
 </script>
-