chore(deps-dev): bump postcss from 6.0.23 to 8.4.31

Bumps [postcss](https://github.com/postcss/postcss) from 6.0.23 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
...

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

package.json

@@ -26,7 +26,7 @@
     "babel-eslint": "^10.1.0",
     "eslint-config-rollup": "^0.1.0",
     "parcel-plugin-metaimage": "^1.2.3",
-    "postcss": "6.0.23",
+    "postcss": "8.4.31",
     "postcss-cli": "^7.1.1",
     "postcss-cssnext": "^3.1.0",
     "postcss-discard-comments": "^4.0.2",