This is the normal standard way in C, C++ as well as Java and there's no
longer any reason for ICU to be different. The various internal macros
providing custom boolean constants can all be deleted and code as well
as documentation can be updated to use lowercase true/false everywhere.
Remove the ICU macros ATOMIC_INT32_T_INITIALIZER and U_INITONCE_INITIALIZER,
which made use of C++ ATOMIC_VAR_INIT, which has been removed from C++20.
With modern C++ features being available, these macros no longer served
any real need.
