Bump postcss from 8.3.8 to 8.3.9

Bumps [postcss](https://github.com/postcss/postcss) from 8.3.8 to 8.3.9.
- [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/8.3.8...8.3.9)

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

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

package.json

@@ -60,7 +60,7 @@
     "jest": "^26.6.3",
     "mini-css-extract-plugin": "^2.3.0",
     "optimize-css-assets-webpack-plugin": "^6.0.1",
-    "postcss": "^8.3.8",
+    "postcss": "^8.3.9",
     "postcss-loader": "^6.1.1",
     "prettier": "^2.4.1",
     "prettier-webpack-plugin": "^1.2.0",