Replace polyfill.io (alshedivat#2241)

The PR replaces `polyfill.io` inside the template with
`cdnjs.cloudflare.com/polyfill`.

---

`polyfill.io` was acquired by **a China-based CDN company** "Funnull",
see [the announcement from the `polyfill.io` domain owner's
Twitter](https://x.com/JakeDChampion/status/1761315227008643367) 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](https://twitter.com/triblondon/status/1761852117579427975). And
since the acquisition, numerous issues have emerged
(polyfillpolyfill/polyfill-service#2835,
polyfillpolyfill/polyfill-service#2838,
alist-org/alist#6100), rendering the
`polyfill.io` service **extremely unstable**. Since then, Fastly
([Announcement](https://community.fastly.com/t/new-options-for-polyfill-io-users/2540))
and Cloudflare
([Announcement](https://blog.cloudflare.com/polyfill-io-now-available-on-cdnjs-reduce-your-supply-chain-risk))
has hosted their own instances of `polyfill.io` service.

_includes/scripts/mathjax.liquid

@@ -13,5 +13,5 @@
     id="MathJax-script"
     src="https://cdn.jsdelivr.net/npm/mathjax@{{ site.mathjax.version }}/es5/tex-mml-chtml.js"
   ></script>
-  <script defer src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
+  <script defer src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>
 {% endif %}