[electrum] update server response message for excessive fee

Summary: This message was changed in v0.24.7 (see release notes).

Test Plan: I tested that this is indeed the error message now returned by the validation logic in D17370

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17371