Bump postcss from 8.5.2 to 8.5.3 in /src/sphinx_antsibull_ext/css (#378)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.2 to 8.5.3.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.2...8.5.3)

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

Signed-off-by: dependabot[bot] <[email protected]>
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": "^10.4.20",
     "cssnano": "^7.0.6",
-    "postcss": "^8.5.2",
+    "postcss": "^8.5.3",
     "postcss-cli": "^11.0.0",
     "sass": "^1.85.0"
   }