Bump postcss from 8.4.17 to 8.4.18 in /assets

Bumps [postcss](/~https://github.com/postcss/postcss) from 8.4.17 to 8.4.18.
- [Release notes](/~https://github.com/postcss/postcss/releases)
- [Changelog](/~https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.17...8.4.18)

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

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

assets/package.json

@@ -30,7 +30,7 @@
     "hard-source-webpack-plugin": "^0.13.1",
     "mini-css-extract-plugin": "^2.6.1",
     "node-sass-glob-importer": "^5.3.2",
-    "postcss": "^8.4.17",
+    "postcss": "^8.4.18",
     "sass": "^1.55.0",
     "sass-loader": "^13.1.0",
     "terser-webpack-plugin": "^5.3.6",