Bump postcss from 8.4.20 to 8.4.21 (#23019)

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:production
  update-type: version-update:semver-patch
...

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

package.json

@@ -84,7 +84,7 @@
     "object.values": "^1.1.6",
     "path-complete-extname": "^1.0.0",
     "pg": "^8.5.0",
-    "postcss": "^8.4.20",
+    "postcss": "^8.4.21",
     "postcss-loader": "^3.0.0",
     "promise.prototype.finally": "^3.1.4",
     "prop-types": "^15.8.1",