polyfill.io library is no longer considered safe to use

I think this priority and issue tag makes sense.

Since it's about a 3rd party library this can be fixed in public without a security advisory, but should ideally be addressed quickly with a code change and new release(s).

Since it's only one change, but to avoid accidentally using the wrong CDN for the files, I have updated the CDN library to use polyfill-fastly.io. Thanks.

Generally looks good to me. Thanks.