Update electron requirement from ^5.0.4 to ^6.0.0 in /app

Updates the requirements on [electron](https://github.com/electron/electron) to permit the latest version.
- [Release notes](https://github.com/electron/electron/releases)
- [Commits](https://github.com/electron/electron/compare/v5.0.4...v6.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

app/package.json

@@ -65,7 +65,7 @@
     "font-manager": "^0.3.0"
   },
   "devDependencies": {
-    "electron": "^5.0.4",
+    "electron": "^6.0.0",
     "electron-builder": "^21.1.5",
     "readline-sync": "1.4.9"
   },