Use chrono Calendars and Time Zones (P0355R7, P1466R3) when available
instead of the `date` library.
Verified with a patched build of a recent GCC 13 snapshot.
The structure was used to pass the locale instance to the date
formatter. All the supported versions of `fmt` are passing the locale
parameter via `FormatContext.locale()` so we can remove the struct and
simplify the code.
While we at it, drop `date::make_zoned` in favor of CTAD on a
`date::zoned_time` constructor.
