Update dependency postcss-custom-properties to v11

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
postcss-custom-properties	10.0.0 -> 11.0.0

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Release Notes

postcss/postcss-custom-properties

### [`v11.0.0`](https://togithub.com/postcss/postcss-custom-properties/blob/master/CHANGELOG.md#​1100-January-12-2021) [Compare Source](https://togithub.com/postcss/postcss-custom-properties/compare/10.0.0...11.0.0) - Added: Support for PostCSS v8.

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