Use C++20's time zone support in std::chrono by default if supported.
C++ compiler and STL implementations are getting C++20 feature complete, so we can look into replacing the
date library with it. This would be especially nice on Windows, since using time zones on Windows would not require extra setup anymore.
This was done in issue #8595 similarly to how we do
- Have a CMake option to select the implementation, defaulting to the STL implementation if supported
- Introduce a
Wt::cpp20::chrononamespace that is aliased to
WLocalDateTime may no longer be necessary. We could look into adding overrides in the right places, so that their use can be avoided if desired.