Bump postcss from 8.4.18 to 8.4.31

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.18 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@8.4.18...8.4.31)

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

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -32,7 +32,7 @@
     "gatsby-source-filesystem": "^3.14.0",
     "gatsby-transformer-remark": "^4.11.0",
     "gatsby-transformer-sharp": "^3.14.0",
-    "postcss": "^8.4.18",
+    "postcss": "^8.4.31",
     "prismjs": "^1.29.0",
     "react": "^17.0.1",
     "react-dom": "^17.0.2",