Downgrade to electron 20.3.4 to fix crash on call hangup (#459)

Co-authored-by: t3chguy <t3chguy@users.noreply.github.com>
Co-authored-by: ElementRobot <releases@riot.im>
Fixes https://github.com/vector-im/element-web/issues/23783
Fixes undefined

package.json

@@ -70,7 +70,7 @@
     "babel-jest": "^28.1.3",
     "chokidar": "^3.5.2",
     "detect-libc": "^1.0.3",
-    "electron": "^20",
+    "electron": "20.3.4",
     "electron-builder": "^23.6.0",
     "electron-builder-squirrel-windows": "^23.6.0",
     "electron-devtools-installer": "^3.1.1",