Update dependency postcss to v8.4.21 - abandoned

This MR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	8.4.19 -> 8.4.21

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Release Notes

postcss/postcss

v8.4.21

Compare Source

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

v8.4.20

Compare Source

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

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