build(deps): bump postcss and next

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates
ancestor dependency [next](https://github.com/vercel/next.js). These dependencies
need to be updated together.

Updates `postcss` from 8.4.14 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@8.4.14...8.4.31)

Updates `next` from 13.5.2 to 13.5.4
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v13.5.2...v13.5.4)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: next
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>