Bump cssnano from 7.0.3 to 7.0.4 in /src/sphinx_antsibull_ext/css (#294)

Bumps [cssnano](https://github.com/cssnano/cssnano) from 7.0.3 to 7.0.4.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@7.0.3...cssnano@7.0.4)

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

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

src/sphinx_antsibull_ext/css/package.json

@@ -1,7 +1,7 @@
 {
   "dependencies": {
     "autoprefixer": "^10.4.19",
-    "cssnano": "^7.0.3",
+    "cssnano": "^7.0.4",
     "postcss": "^8.4.39",
     "postcss-cli": "^11.0.0",
     "sass": "^1.77.6"