Update dependency gulp-postcss to v10

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gulp-postcss	8.0.0 -> 10.0.0

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Release Notes

postcss/gulp-postcss (gulp-postcss)

v10.0.0

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Same changes as https://github.com/postcss/gulp-postcss/releases/tag/9.1.0, published with the major version bump because of a breaking change.

v9.1.0: (deprecated)

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Deprecated, republished as 10.0.0 to follow semver, because of the breaking change to drop support for node <18

• Bump postcss-load-config to ^5.0.0
• Ensure options are passed to plugins when using postcss.config.js #​170
• Update deps
• Drop support for node <18
• Add flake.nix for local dev with nix develop

v9.0.1

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• Bump postcss-load-config to ^3.0.0

v9.0.0

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• Bump PostCSS to 8.0
• Drop Node 6 support
• PostCSS is now a peer dependency

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