Update the signed message prefix to eCash

Summary: Rebrand the message prefix to eCash rather than bitcoin. Since I'm not aware of any use case for keeping the legacy prefix I added to compatibility flag, but only a release note.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10043