Bump postcss in /src/sphinx_antsibull_ext/css in the npm group (#456)

Bumps the npm group in /src/sphinx_antsibull_ext/css with 1 update: [postcss](https://github.com/postcss/postcss).


Updates `postcss` from 8.5.8 to 8.5.9
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.8...8.5.9)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.9
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: npm
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/sphinx_antsibull_ext/css/package-lock.json

@@ -2,7 +2,7 @@
   "dependencies": {
     "autoprefixer": "^10.4.27",
     "cssnano": "^7.1.4",
-    "postcss": "^8.5.8",
+    "postcss": "^8.5.9",
     "postcss-cli": "^11.0.1",
     "sass": "^1.99.0"
   }