Bump unified from 8.4.2 to 9.0.0 (#7904)
Bumps [unified](https://github.com/unifiedjs/unified) from 8.4.2 to 9.0.0.
- [Release notes](https://github.com/unifiedjs/unified/releases)
- [Changelog](https://github.com/unifiedjs/unified/blob/master/changelog.md)
- [Commits](https://github.com/unifiedjs/unified/compare/8.4.2...9.0.0)
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