Update dependency postcss to v8.4.18

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	8.4.17 -> 8.4.18

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Release Notes

postcss/postcss

### [`v8.4.18`](https://togithub.com/postcss/postcss/blob/HEAD/CHANGELOG.md#​8418) [Compare Source](https://togithub.com/postcss/postcss/compare/8.4.17...8.4.18) - Fixed an error on `absolute: true` with empty `sourceContent` (by Rene Haas).

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