refactor: Replace global find_value function with UniValue::find_value method
Summary:
These warning are very noisy and annoying when working on large refactors, hiding the useful information from the developer.
Backport of core#27605.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D15097