build(deps-dev): bump postcss from 8.3.0 to 8.3.4

Bumps [postcss](https://github.com/postcss/postcss) from 8.3.0 to 8.3.4.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.3.0...8.3.4)

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

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

Author: dependabot[bot]

package-lock.json

@@ -19,7 +19,7 @@
     "eslint": "^7.18",
     "hugo-bin": "^0.68.0",
     "markdownlint-cli": "^0.26",
-    "postcss": "^8.2.6",
+    "postcss": "^8.3.4",
     "postcss-cli": "^8.3.1",
     "postcss-import": "^14.0.0",
     "prettier": "^2.2.1",