Update dependency postcss to v8.5.6

This MR contains the following updates:

Package	Change	Age	Confidence
postcss (source)	8.5.1 → 8.5.6

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Release Notes

postcss/postcss (postcss)

v8.5.6

Compare Source

• Fixed ContainerWithChildren type discriminating (by @​Goodwine).

v8.5.5

Compare Source

• Fixed package.json→exports compatibility with some tools (by @​JounQin).

v8.5.4

Compare Source

• Fixed Parcel compatibility issue (by @​git-sumitchaudhary).

v8.5.3

Compare Source

• Added more details to Unknown word error (by @​hiepxanh).
• Fixed types (by @​romainmenke).
• Fixed docs (by @​catnipan).

v8.5.2

Compare Source

• Fixed end position of rules with semicolon (by @​romainmenke).

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