This is a cleaning followup for #11456.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Begins the transition to using fmt for string formatting where
applicable. Given fmt supports formatting std::string instances out of
the box, we can remove now-unnecessary calls to .c_str() and .data().

Note that this change does not touch the actual logging subsystem aside
from converting the final StringFromFormat call in the process over to
fmt::format. Given our logging system is heavily used throughout the
entire codebase, and converting that over will be quite a large change
by itself, this will be tackled near the end of the conversion process.

It was only ever passed nullptr, and even then, nothing was actually
done with the parameter.

Amends the function names to follow our coding style.