[herald] shorter identifier for anon-ish miner

Summary: We see this miner a lot. Since we have no other miners identified only by their payout scripts, there is no need to use more than 1 byte of data here.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15136