Bump postcss from 8.4.19 to 8.4.20

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package.json

@@ -26,7 +26,7 @@
     "grunt-stylelint": "0.16.0",
     "grunt-terser": "2.0.0",
     "load-grunt-tasks": "5.1.0",
-    "postcss": "8.4.19",
+    "postcss": "8.4.20",
     "sass": "1.56.2",
     "stylelint": "13.13.1",
     "stylelint-order": "4.1.0",