Bump postcss from 8.3.9 to 8.3.11 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.3.9 to 8.3.11.
- [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/8.3.9...8.3.11)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

frontend/package.json

@@ -15,7 +15,7 @@
         "laravel-mix": "^6",
         "lodash": "^4.17.21",
         "lodash.clonedeep": "^4.5.0",
-        "postcss": "^8.1.14",
+        "postcss": "^8.3.11",
         "resolve-url-loader": "^4.0.0",
         "sass": "^1.43.2",
         "sass-loader": "^12.2.0",