chore(deps-dev): bump postcss (#860)

Bumps the all group with 1 update in the /site directory: [postcss](https://github.com/postcss/postcss).


Updates `postcss` from 8.5.14 to 8.5.15
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.5.14...8.5.15)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: all
...

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