fix: upgrade postcss from 8.5.1 to 8.5.6

Snyk has created this PR to upgrade postcss from 8.5.1 to 8.5.6.

See this package in npm:
postcss

See this project in Snyk:
https://app.snyk.io/org/colhodm/project/98dfc0ca-c297-4f7c-9aaa-1878634708d1?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

tools/server/webui/package-lock.json

@@ -22,7 +22,7 @@
     "highlight.js": "^11.10.0",
     "katex": "^0.16.15",
     "pdfjs-dist": "^5.2.133",
-    "postcss": "^8.4.49",
+    "postcss": "^8.5.6",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-dropzone": "^14.3.8",