chore(deps-dev): bump postcss from 8.5.4 to 8.5.6

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.4 to 8.5.6.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.4...8.5.6)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

package-lock.json

@@ -61,7 +61,7 @@
     "grunt-version": "^3.0.1",
     "grunt-wp-readme-to-markdown": "^2.1.0",
     "npm-run-all": "^4.1.5",
-    "postcss": "^8.5.4",
+    "postcss": "^8.5.6",
     "replace-in-file": "^7.1.0",
     "semantic-release": "^19.0.5",
     "semantic-release-slack-bot": "^4.0.2",