Bump postcss from 8.4.21 to 8.4.31 (#805)

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](https://github.com/postcss/postcss/compare/8.4.21...8.4.31)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Emruz Hossain <hossainemruz@gmail.com>

package.json

@@ -61,7 +61,7 @@
     "mermaid": "^9.2.1",
     "plyr": "^3.7.2",
     "popper.js": "^1.16.1",
-    "postcss": "^8.4.21",
+    "postcss": "^8.4.31",
     "postcss-cli": "^8.3.1"
   },
   "homepage": "https://github.com/hugo-toha/toha#readme",