Bump postcss from 8.4.19 to 8.4.21 (#132)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.19 to 8.4.21.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.19...8.4.21)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -62,7 +62,7 @@
     "normalise": "^10.0.0",
     "normalize.css": "^8.0.1",
     "null-loader": "^4.0.1",
-    "postcss": "^8.4.19",
+    "postcss": "^8.4.21",
     "postcss-import": "^15.1.0",
     "postcss-loader": "^7.0.2",
     "postcss-preset-env": "^7.8.3",