[electrum] release notification for 5.2.8

Summary: And make the "Test run" message more explicit

Test Plan:
./electrum-abc --test-release-notification
Wait for the notification or trigger it via menu About > Check for updates
Check that the url in the notification has the latest version for download (check that the website is deployed)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14617