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