Update dependency postcss to v8.2.4

This MR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	patch	8.2.1 -> 8.2.4

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Release Notes

postcss/postcss

v8.2.4

Compare Source

• Added plugin name to postcss.plugin() warning (by Tom Williams).
• Fixed docs (by Bill Columbia).

v8.2.3

Compare Source

• Fixed JSON.stringify(Node[]) support (by Niklas Mischkulnig).

v8.2.2

Compare Source

• Fixed CSS-in-JS support (by James Garbutt).
• Fixed plugin types (by Ludovico Fischer).
• Fixed Result#warn() types.

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