Bump postcss from 8.4.6 to 8.4.31 in /web/vtadmin

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.6 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@8.4.6...8.4.31)

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

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

web/vtadmin/package.json

@@ -84,7 +84,7 @@
     "@types/lodash-es": "^4.17.4",
     "autoprefixer": "^10.4.2",
     "msw": "^0.36.8",
-    "postcss": "^8.4.6",
+    "postcss": "^8.4.31",
     "prettier": "^2.2.1",
     "protobufjs": "^6.11.3",
     "serve": "^13.0.2",