fix(deps): update dependency rehype-slug to v6

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This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
rehype-slug	5.1.0 -> 6.0.0

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Release Notes

rehypejs/rehype-slug (rehype-slug)

### [`v6.0.0`](https://togithub.com/rehypejs/rehype-slug/releases/tag/6.0.0) [Compare Source](https://togithub.com/rehypejs/rehype-slug/compare/5.1.0...6.0.0) ##### Change - [`295ac26`](https://togithub.com/rehypejs/rehype-slug/commit/295ac26) Update `@types/hast`, `unified`, utilities, etc **migrate**: update too - [`12f76f8`](https://togithub.com/rehypejs/rehype-slug/commit/12f76f8) Change to require Node.js 16 **migrate**: update too - [`6855bcc`](https://togithub.com/rehypejs/rehype-slug/commit/6855bcc) Change to use `exports` **migrate**: don’t use private APIs **Full Changelog**: https://github.com/rehypejs/rehype-slug/compare/5.1.0...6.0.0

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