fix(deps): update webpack-cli to v6

Agent-Logs-Url: https://github.com/nextcloud/mail/sessions/bb3b88ac-3caa-42e3-87e3-fa96b6f2f1ad

Co-authored-by: ChristophWurst <1374172+ChristophWurst@users.noreply.github.com>

Author: Copilot

package-lock.json

@@ -109,7 +109,7 @@
     "vue-loader": "^15.11.1",
     "vue-template-compiler": "^2.7.16",
     "webpack": "^5.106.2",
-    "webpack-cli": "^5.1.4",
+    "webpack-cli": "^6.0.1",
     "webpack-merge": "^5.10.0"
   },
   "engines": {