fix: ui/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-IP-12704893
- https://snyk.io/vuln/SNYK-JS-IP-12761655

Author: snyk-bot

ui/package.json

@@ -75,7 +75,7 @@
     "unidiff": "^1.0.2",
     "webpack": "^4.44.2",
     "webpack-cli": "^3.3.12",
-    "webpack-dev-server": "^3.11.0"
+    "webpack-dev-server": "^4.8.0"
   },
   "resolutions": {
     "@types/react": "16.9.3",