Bump postcss from 8.4.20 to 8.4.21

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.20 to 8.4.21.
- [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/8.4.20...8.4.21)

---
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

@@ -19,7 +19,7 @@
         "eslint": "^8.30.0",
         "eslint-webpack-plugin": "^3.2.0",
         "mini-css-extract-plugin": "^2.7.2",
-        "postcss": "^8.4.20",
+        "postcss": "^8.4.21",
         "postcss-combine-duplicated-selectors": "^10.0.2",
         "postcss-combine-media-query": "^1.0.1",
         "postcss-import": "^15.1.0",