Update dependency postcss to v8.4.21

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	8.4.20 -> 8.4.21

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Release Notes

postcss/postcss

### [`v8.4.21`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#​8421) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.20...8.4.21) - Fixed `Input#error` types (by Aleks Hudochenkov).

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Configuration

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♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

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