[backport#14599] Use functions guaranteed to be locale independent

Summary:
Use IsDigit(...) instead of std::isdigit
Use ToLower(...) instead of std::tolower
Use IsSpace(...) instead of boost::is_space
Update KNOWN_VIOLATIONS: Remove fixed violations

Backport of Core PR14599

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6294