Bump postcss and autoprefixer in /themes/docsy/userguide

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [autoprefixer](https://github.com/postcss/autoprefixer). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.31)

Updates `autoprefixer` from 9.8.6 to 10.4.16
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@9.8.6...10.4.16)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: autoprefixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

themes/docsy/userguide/package.json

@@ -18,7 +18,7 @@
   "homepage": "https://github.com/google#readme",
   "dependencies": {},
   "devDependencies": {
-    "autoprefixer": "^9.5.0",
+    "autoprefixer": "^10.4.16",
     "postcss-cli": "^10.0.0"
   }
 }