Rename sigop to sigcheck in getblocktemplate
Summary: Add a deprecation notice, and a flag to print the legacy field if needed.
Test Plan:
ninja all check-all bitcoin-cli help getblocktemplate
Check the RPC help is showing the deprecation properly and the way to get the previous behavior back.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12811