util: refactor upper/lowercase functions

Summary:
This includes renaming Downcase() to ToLower() and make it return a string rather than modify referenced arg.
Also adds ToUpper() string version.

This is a backport of Core PR16566

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6376