Bump postcss from 8.4.21 to 8.4.31 in /packages/graphiql (#3482)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.21 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@8.4.21...8.4.31)

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

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

Author: dependabot[bot]

packages/graphiql/package.json

@@ -79,7 +79,7 @@
     "html-webpack-plugin": "^5.5.0",
     "identity-obj-proxy": "^3.0.0",
     "mini-css-extract-plugin": "^2.7.2",
-    "postcss": "8.4.21",
+    "postcss": "8.4.31",
     "postcss-import": "15.1.0",
     "postcss-loader": "7.0.2",
     "postcss-preset-env": "^8.0.1",