Bump postcss in /slides in the npm_and_yarn group across 1 directory

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


Updates `postcss` from 8.5.6 to 8.5.15
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.6...8.5.15)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: indirect
  dependency-group: npm_and_yarn
...

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

Author: dependabot[bot]