Bump postcss from 8.2.8 to 8.2.13 in /assets

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.8 to 8.2.13.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.2.8...8.2.13)

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

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

Author: dependabot[bot]

assets/package-lock.json

@@ -23,7 +23,7 @@
     "mini-css-extract-plugin": "^1.3.9",
     "npm": "^6.14.5",
     "optimize-css-assets-webpack-plugin": "^5.0.4",
-    "postcss": "^8.2.8",
+    "postcss": "^8.2.13",
     "postcss-import": "^14.0.0",
     "postcss-loader": "^5.2.0",
     "tailwindcss": "^2.1.1",