build(deps): bump postcss from 8.4.19 to 8.4.20 (#4966)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.19 to 8.4.20.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.19...8.4.20)

---
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

@@ -51,7 +51,7 @@
     "ncp": "^2.0.0",
     "node-version-data": "^1.1.0",
     "npm-run-all": "^4.1.5",
-    "postcss": "^8.4.18",
+    "postcss": "^8.4.20",
     "sass": "^1.56.1",
     "semver": "^7.3.8",
     "strftime": "^0.10.1"