Rename sigOps to sigCheks in the policy files

Summary:
Similar to D12785, but for the policy variables/functions.

Depends on D12785 and D12786.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12787