Bump postcss from 8.4.29 to 8.4.31 in /docs/vercel (#1186)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.29 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.29...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

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