Update dependency postcss-mixins to v9

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss-mixins	^6.2.3 -> ^9.0.0

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Release Notes

postcss/postcss-mixins (postcss-mixins)

v9.0.4

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• Moved to postcss-simple-vars 7.

v9.0.3

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• Fixed hot reload (by Andrey Grandilevskiy).
• Fixed Windows support (by Andrey Grandilevskiy).

v9.0.2

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• Fixed at-rules in @mixin-content in function mixins (by Vasily Polovnyov).
• Updated postcss-js.

v9.0.1

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• Reduced package size.

v9.0.0

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v8.1.0

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v8.0.0

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v7.0.3

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• Fixed file pattern on Windows (by Gene Alyson Fortunado Torcende).

v7.0.2

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• Fixed postcss-js loading (by Paul Welsh).

v7.0.1

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• Added funding links.

v7.0.0

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This change is marked as an internal change (Task), so will not be included in the changelog.