[electrum] start supporting oa1:xec and stop supporting oa1:bch as openalias prefix

Summary: This was never updated after the fork, and nobody complained. It is probably still worth supporting OpenAliases, so fix it.

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14532