[electrum] push notification for release 5.2.12

Summary: As per title.

Test Plan:
./electrum-abc --test-release-notification
Menu Help > Check for updates

Change a random value in the middle of the signature, redo the test, check that it fails

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15907