build(deps): bump postcss and next in /website (#196)

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 8.4.5 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@8.4.5...8.4.31)

Updates `next` from 12.1.6 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@v12.1.6...v13.5.4)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]