Bump postcss from 8.4.28 to 8.4.31 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.28 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.4.28...8.4.31)

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

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

frontend/package.json

@@ -18,7 +18,7 @@
 		"eslint": "^8.28.0",
 		"eslint-config-prettier": "^8.5.0",
 		"eslint-plugin-svelte": "^2.30.0",
-		"postcss": "^8.4.28",
+		"postcss": "^8.4.31",
 		"prettier": "^2.8.0",
 		"prettier-plugin-svelte": "^2.10.1",
 		"svelte": "^4.0.5",