fix: ui/package.json & ui/yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-CROSSSPAWN-8303230

Author: snyk-bot

ui/package.json

@@ -74,8 +74,8 @@
     "typescript": "^4.0.3",
     "unidiff": "^1.0.2",
     "webpack": "^4.44.2",
-    "webpack-cli": "^3.3.12",
-    "webpack-dev-server": "^3.11.0"
+    "webpack-cli": "^4.0.0",
+    "webpack-dev-server": "^4.6.0"
   },
   "resolutions": {
     "@types/react": "16.9.3",