Update dependency postcss to ^8.1.8

Created by: renovate[bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss (source)	^8.1.1 -> ^8.1.8

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Release Notes

postcss/postcss

v8.1.8

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• Fix LazyResult type compatibility with Promise (by Ludovico Fischer).
• Use HTTPS links in documentation.

v8.1.7

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• Fix import support in TypeScript (by Remco Haszing).

v8.1.6

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• Reverted package.exports Node.js 15 fix.

v8.1.5

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• Fixed Node.js 15 warning (by 沈鸿飞).

v8.1.4

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• Fixed TypeScript definition (by Arthur Petrie).

v8.1.3

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• Added package.types.

v8.1.2

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• Fixed API docs (by Arthur Petrie).
• Improved plugin guide (by Yunus Gaziev).
• Prepared code base for Deno support (by Oscar Otero).

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