Update dependency postcss-combine-duplicated-selectors to v9

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
postcss-combine-duplicated-selectors	devDependencies	major	8.1.0 -> 9.0.0

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Release Notes

ChristianMurphy/postcss-combine-duplicated-selectors

### [`v9.0.0`](https://togithub.com/ChristianMurphy/postcss-combine-duplicated-selectors/releases/v9.0.0) [Compare Source](https://togithub.com/ChristianMurphy/postcss-combine-duplicated-selectors/compare/v8.1.0...v9.0.0) Node 8 may still work, but it is no longer a part of the CI tests and is not officially supported. Node 14 is now officially supported.

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