fix: update dependency postcss to v8.4.1

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	8.4.0 -> 8.4.1

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Release Notes

postcss/postcss

### [`v8.4.1`](https://togithub.com/postcss/postcss/blob/master/CHANGELOG.md#​841) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.0...8.4.1) - Fixed `Stringifier` types (by James Garbutt).

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