[C++] Remove the vendored datetime fallback when toolchain support permits

[rok]

Describe the enhancement requested

#51216 makes std::chrono the default datetime backend and removes vendored datetime dependencies from standard-backend binaries. However most builds still default to vendored date.h library. The vendored fallback must remain for unsupported standard libraries.

This requires:

• Raising the minimum Linux libstdc++ version to GCC 16.2 or newer.
• Apple libc++ providing working C++20 timezone and formatting APIs.
• Migrating Rtools40 and MSYS2 Clang64 builds.
• Validating Emscripten, and other supported targets.
• Retiring the ARROW_USE_STD_CHRONO=0 override.

Once all supported toolchains use std::chrono, remove the vendored sources and build plumbing (e.g. reload_tzdb). The Windows text-tzdb configuration APIs, including the deprecated Python API and R initialization, can also be removed once no supported Windows build uses the fallback.

Component(s)

C++

[Adarsh-Me]

take