Bump the npm group in /src/sphinx_antsibull_ext/css with 2 updates (#458)

Bumps the npm group in /src/sphinx_antsibull_ext/css with 2 updates: [cssnano](https://github.com/cssnano/cssnano) and [postcss](https://github.com/postcss/postcss).


Updates `cssnano` from 7.1.5 to 7.1.7
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@7.1.5...cssnano@7.1.7)

Updates `postcss` from 8.5.10 to 8.5.12
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.10...8.5.12)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-version: 7.1.7
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: npm
- dependency-name: postcss
  dependency-version: 8.5.12
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: npm
...

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Author: dependabot[bot]