Upgrade electron-builder to 21.2.0

This is meant to help with:

* macOS packaging for Electron 6 (https://github.com/vector-im/riot-web/issues/10473)
* Linux desktop file should not have quotes (https://github.com/vector-im/riot-web/issues/6421)

It did not help with:

* Linux sandbox for Electron 5 (https://github.com/vector-im/riot-web/issues/10509)

Fixes https://github.com/vector-im/riot-web/issues/6421
Part of https://github.com/vector-im/riot-web/issues/10473

package.json

@@ -108,8 +108,8 @@
     "cpx": "^1.3.2",
     "cross-env": "^4.0.0",
     "css-loader": "^2.1.0",
-    "electron-builder": "^20.44.4",
-    "electron-builder-squirrel-windows": "^21.0.6",
+    "electron-builder": "^21.2.0",
+    "electron-builder-squirrel-windows": "^21.2.0",
     "electron-devtools-installer": "^2.2.4",
     "eslint": "^5.8.0",
     "eslint-config-google": "^0.7.1",