Update to the latest Electron & enable now-stable integrity flag

This is the last Electron step from the security review - doesn't
provide hard guarantees, but does ensure that our code signing cert is
locked to the actual code we're running which is a good start.

Author: pimterry

package-lock.json

@@ -135,6 +135,7 @@
       "enableNodeOptionsEnvironmentVariable": false,
       "enableNodeCliInspectArguments": false,
       "onlyLoadAppFromAsar": true,
+      "enableEmbeddedAsarIntegrityValidation": true,
       "loadBrowserProcessSpecificV8Snapshot": false,
       "grantFileProtocolExtraPrivileges": false,
       "resetAdHocDarwinSignature": false
@@ -159,7 +160,7 @@
     "@types/targz": "^1.0.0",
     "@types/yargs": "^15.0.3",
     "cross-env": "^7.0.3",
-    "electron": "^37.10.3",
+    "electron": "^39.2.7",
     "electron-builder": "^26.0.2",
     "lodash": "^4.17.21",
     "node-fetch": "^2.6.7",