[electrum] bump the version to 5.2.6 and add release notes

Summary: As per title.

Test Plan:
python setup.py --version

Run ./electrum-abc and verify the correct version is displayed in the application (main window title and menu Help > About)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14301