Bump postcss and web-ext (#60)

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.33 and updates ancestor dependency [web-ext](https://github.com/mozilla/web-ext). These dependencies need to be updated together.


Updates `postcss` from 8.4.29 to 8.4.33
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.29...8.4.33)

Updates `web-ext` from 7.8.0 to 7.11.0
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@7.8.0...7.11.0)

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

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Author: dependabot[bot]