[backport#15139] util: Make ToLower and ToUpper take a char

Summary:
Unfortunately, std::string elements are (bare) chars. As these
are the most likely type to be passed to these functions, make them use
char instead of unsigned char. This avoids some casts.

https://github.com/bitcoin/bitcoin/pull/15139/commits/332b3dd7c156daa08df347dc3359f0ee99ce4baa

Depends on D6294

Concludes backport of Core PR15139

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6295