Bump postcss from 7.0.39 to 8.4.39 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.39 to 8.4.39.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.39)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

frontend/package.json

@@ -29,7 +29,7 @@
     "minimatch": "^9.0.0",
     "muicss": "^0.10.3",
     "normalize.css": "^8.0.1",
-    "postcss": "^8.4.38",
+    "postcss": "^8.4.39",
     "pug-lint-vue": "^0.4.0",
     "seedrandom": "^3.0.5",
     "vue": "~3.4.0",