Bump postcss from 8.4.21 to 8.4.31 in /htdocs

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.21 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@8.4.21...8.4.31)

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

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

Author: dependabot[bot]

htdocs/package-lock.json

@@ -23,7 +23,7 @@
         "autoprefixer": "^10.4.13",
         "chokidar": "^3.5.3",
         "cssnano": "^6.0.0",
-        "postcss": "^8.4.21",
+        "postcss": "^8.4.31",
         "rtlcss": "^4.0.0",
         "sass": "^1.57.1",
         "terser": "^5.16.1",