build(deps): bump postcss and next

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.26 and updates ancestor dependency [next](https://github.com/vercel/next.js). These dependencies need to be updated together.


Updates `postcss` from 8.5.6 to 8.5.26
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.6...8.5.26)

Updates `next` from 15.5.21 to 16.3.4
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](vercel/next.js@v15.5.21...v16.3.4)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 16.3.0
  dependency-type: direct:production
- dependency-name: postcss
  dependency-version: 8.5.25
  dependency-type: indirect
...

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

Author: dependabot[bot]