Bump postcss from 8.4.31 to 8.5.10 in /webpack-plugin (#5306)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.31 to 8.5.10.
- [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/8.4.31...8.5.10)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.10
  dependency-type: indirect
...

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