[electrum] rename migrate_data.py to update.py and fix module docstring

Summary:
This module was initially created to migrate ElectronCash wallets, but it no longer does that since https://github.com/Bitcoin-ABC/ElectrumABC/pull/258
Update the name and docstring to what the module now does.

Depends on D16875

Test Plan: bump the version number in version.py, start the application and check for the log message INFO:electrumabc.update:Updating configuration file 5.3.0 🠚 5.3.1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16883