Output proper coinbase value in getblocktemplate once the new coinbase rule is active

Summary: This ensures that the coinbase value is properly reported by the getblocktemplate RPC.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox, majcosta

Reviewed By: #bitcoin_abc, jasonbcox, majcosta

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7201