docs: no longer recommend `polyfill.io`

[SukkaW]

Checklist

• [x] I have ensured my pull request is not behind the main or master branch of the original repository.
• [x] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
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Replace the polyfill.io with cdnjs.cloudflare.com/polyfill in the README and the docs:

polyfill.io was acquired by a China-based CDN company "Funnull", see the announcement from the polyfill.io domain owner's Twitter and https://github.com/polyfillpolyfill/polyfill-service/issues/2834. Despite Funnull's claims of operating in the United States, the predominance of Simplified Chinese on its website suggests otherwise, and it turns out that "Funnull" is notorious for providing service for the betting and pornography industries.

The original creator of the polyfill.io has voiced his concern on Twitter. And since the acquisition, numerous issues have emerged (https://github.com/polyfillpolyfill/polyfill-service/issues/2835, https://github.com/polyfillpolyfill/polyfill-service/issues/2838, https://github.com/alist-org/alist/issues/6100), rendering the polyfill.io service extremely unstable. Since then, Fastly (Announcement) and Cloudflare (Announcement) has hosted their own instances of polyfill.io service.