Update polyfill link

[iosh]

The usage of polyfill.io is not secure. Please refer to the following link for details : https://sansec.io/research/polyfill-supply-chain-attack

So switch to Fastly. More information can be found here : https://community.fastly.com/t/new-options-for-polyfill-io-users/2540

[google-cla[bot]]

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[niooss-ledger]

Hello, [I'm just someone who searched polyfill.io on GitHub Search and found this Pull Request, I'm not related to this project] Your Pull Request only modifies README.md, but its content seems to be generated from site/src/api.njk: https://github.com/GoogleChromeLabs/quicklink/blob/cd9ba8e870a48d5f867c6b89c596f324c83789f6/site/src/api.njk#L193-L197

I believe this file should also be updated.

[XhmikosR]

/CC @addyosmani

[XhmikosR]

The links have been removed on main.