Update dependency postcss to v8.4.21

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	8.4.13 -> 8.4.21

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Release Notes

postcss/postcss

v8.4.21

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• Fixed Input#error types (by Aleks Hudochenkov).

v8.4.20

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• Fixed source map generation for childless at-rules like @layer.

v8.4.19

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• Fixed whitespace preserving after AST transformations (by Romain Menke).

v8.4.18

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• Fixed an error on absolute: true with empty sourceContent (by Rene Haas).

v8.4.17

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• Fixed Node.before() unexpected behavior (by Romain Menke).
• Added TOC to docs (by Mikhail Dedov).

v8.4.16

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• Fixed Root AST migration.

v8.4.15

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• Fixed AST normalization after using custom parser with old PostCSS AST.

v8.4.14

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• Print “old plugin API” warning only if plugin was used (by @​zardoy).

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