src: migrate to fmt::formattable for format support

src: migrate to fmt::formattable for format support

Replace custom has_formatter concept with fmtlib's fmt::formattable

- Deprecate usage of fmt::has_formatter
- Resolve compiler warnings with recent fmtlib versions

The custom concept of has_formatter is no longer necessary with
recent updates to the fmtlib library, as fmt::formattable was introduced
in fmtlib v11.0.0. By switching to fmt::formattable, we simplify our
code and stop using the deprecated `fmt::has_formatter` template.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>