Bump postcss from 8.4.21 to 8.4.31 in /web/vtadmin (#14173)

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

Author: vitess-bot[bot]

web/vtadmin/package-lock.json

@@ -89,7 +89,7 @@
     "jsdom": "^21.1.1",
     "msw": "^0.36.8",
     "npm": "^9.6.3",
-    "postcss": "^8.4.6",
+    "postcss": "^8.4.31",
     "prettier": "^2.2.1",
     "protobufjs-cli": "^1.1.1",
     "serve": "^14.2.0",