Bump autoprefixer in /src/sphinx_antsibull_ext/css in the npm group (#380)

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


Updates `autoprefixer` from 10.4.20 to 10.4.21
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@10.4.20...10.4.21)

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

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

@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "autoprefixer": "^10.4.20",
+    "autoprefixer": "^10.4.21",
     "cssnano": "^7.0.6",
     "postcss": "^8.5.3",
     "postcss-cli": "^11.0.0",