Report updated sigops count in mining RPC

Summary: As per title. We now have different limits.

Test Plan:

ninja all check-all

Unortunately, nothing seems to be checking this value is correct but at least nothign's broken and it can be verified manually.

Reviewers: #bitcoin_abc, majcosta, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, majcosta, jasonbcox, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6093