Update electron-builder requirement from ^20.44.4 to ^21.1.5 in /app

Updates the requirements on [electron-builder](https://github.com/electron-userland/electron-builder) to permit the latest version.
- [Release notes](https://github.com/electron-userland/electron-builder/releases)
- [Changelog](https://github.com/electron-userland/electron-builder/blob/master/CHANGELOG.md)
- [Commits](https://github.com/electron-userland/electron-builder/compare/v20.44.4...v21.1.5)

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

app/package.json

@@ -66,7 +66,7 @@
   },
   "devDependencies": {
     "electron": "^5.0.4",
-    "electron-builder": "^20.44.4",
+    "electron-builder": "^21.1.5",
     "readline-sync": "1.4.9"
   },
   "build": {