turns out what we need is a fmt::formatter<sstables::generation_type>
not operator<<(ostream&, sstables::generation_type), as its only use
case is the formatter used by seastar::format().
to specialize fmt::formatter<sstables::generation_type>
* allows us to be one step closer to drop `FMT_DEPRECATED_OSTREAM`
* allows us to customize the way how generation_type is printed by
customizing the format specifier.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12983
f3e6c9168c