Bump postcss from 8.4.16 to 8.4.31 in /fixtures/flight

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.16 to 8.4.31.
- [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/8.4.16...8.4.31

)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:production
...

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