[C++] C++20: use standard calendar / timezone APIs

[pitrou]

Describe the enhancement requested

C++20 finally defines standard APIs for calendar dates and timezones: https://en.cppreference.com/w/cpp/chrono.html#Time_zone

However, some old gcc and clang versions that we might need to support don't support these APIs in full.

So we would need to still our date backport on these compilers.

The main point of doing this would be to (hopefully!) use the system-provided timezone automatically on Windows, while the user currently needs to install a timezone databases themselves.

Component(s)

C++

[pitrou]

@rok

[rok]

Getting rid of windows workarounds would be great indeed. I'll take a look asap.