Remove polyfill.io from demo files - service no longer secure

[cdrini]

See https://fossa.com/blog/polyfill-supply-chain-attack-details-fixes/

Switch to cloudflare's polyfill service, since cloudflare is more reputable/security focussed. See https://blog.cloudflare.com/automatically-replacing-polyfill-io-links-with-cloudflares-mirror-for-a-safer-internet/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.25%. Comparing base (44da86f) to head (8a55cb0). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1351 +/- ## ======================================= Coverage 69.25% 69.25% ======================================= Files 59 59 Lines 5090 5090 Branches 1072 1072 ======================================= Hits 3525 3525 Misses 1538 1538 Partials 27 27 ```

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[cdrini]

Auto-merging; small change and has security implications