Bump postcss and web-ext

Removes [postcss](https://github.com/postcss/postcss). It's no longer used after updating ancestor dependency [web-ext](https://github.com/mozilla/web-ext). These dependencies need to be updated together.


Removes `postcss`

Updates `web-ext` from 7.6.2 to 7.12.0
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@7.6.2...7.12.0)

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

Signed-off-by: dependabot[bot] <[email protected]>