fix(deps): upgrade mocha to 12.0.0-beta-10 for serialize-javascript 7.x

Mocha 11.x still depends on serialize-javascript ^6.0.2 (vulnerable).
Remove npm overrides; use Mocha 12 beta which pulls serialize-javascript
^7.0.2 (resolves to 7.0.5, fixing Dependabot alert #94).

Resolves GHSA-5c6j-r48x-rmvq.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

package.json

@@ -99,7 +99,7 @@
     "express": "^5.2.1",
     "express-ws": "^5.0.2",
     "jsdom": "^29.1.1",
-    "mocha": "^11.7.5",
+    "mocha": "12.0.0-beta-10",
     "mustache": "^4.2.0",
     "node-pty": "^1.2.0-beta.9",
     "nyc": "^18.0.0",
@@ -115,8 +115,5 @@
     "webpack-cli": "^7.0.2",
     "ws": "^8.20.0",
     "xterm-benchmark": "^0.3.2"
-  },
-  "overrides": {
-    "serialize-javascript": "^7.0.5"
   }
 }