Remove mentions of polyfill.io

[Daeloth]

Links to the mentions: https://github.com/pixijs/pixijs.com/blob/a08bd3dc4f018746b32877f3fe05903f020e610f/docs/guides/migrations/v7.md?plain=1#L11 https://github.com/pixijs/pixijs.com/blob/a08bd3dc4f018746b32877f3fe05903f020e610f/versioned_docs/version-7.x/guides/migrations/v7.md?plain=1#L11

Information on polyfill.io: https://x.com/triblondon/status/1761852117579427975 https://github.com/polyfillpolyfill/polyfill-service/issues/2873

Polyfill.io shouldn't be linked to, the domain has been compromised and now links to chinese scams/malware

[Zyie]

thanks for raising this, the links have now been removed!