Bump postcss from 8.4.16 to 8.4.21

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.16 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@8.4.16...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

@@ -110,7 +110,7 @@
     "node-fetch": "^2.6.1",
     "npm-run-all": "^4.0.0",
     "osm-community-index": "~5.5.0",
-    "postcss": "^8.1.1",
+    "postcss": "^8.4.21",
     "postcss-selector-prepend": "^0.5.0",
     "shelljs": "^0.8.0",
     "shx": "^0.3.0",