[electrum] notify users of release 5.4.2

Summary: Trigger the update notification in the app.

Test Plan:
./electrum-abc --test-release-notification

Wait for the notification to show up in the bottom bar, or trigger it via "Check for updates" in the menu

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18120