Replace polyfills link to safe one

[sanak]

We just created the following PR to openlayers side, but there was security news today. https://github.com/openlayers/openlayers/pull/15946

: We noticed the following polyfill.io security news today. https://sansec.io/research/polyfill-supply-chain-attack

And we noticed that there are 2 links in this repository's documents.

This PR replaces the link to safer ones (Fastly or Cloudflare), so reviewing this is helpful.

And there are many links in this repository's examples. https://github.com/search?q=repo%3AViglino%2Fol-ext+polyfill.io&type=code&p=1

It would be nice to replace the link to either of Fastly or Cloudflare one.

[Viglino]

I've removed polyfill (now well suppported).

[Viglino]

Removed from docs

[sanak]

Okay, thanks a lot! 🙇