time_of_day needs C++20 support (not C++17)

[RainerGrimm]

time_of_day need C++20 support (not C++17) time_of_day is an alias for hh_mm_ss<Duration>. Classt emplate argument deduction for alias templates is a C++20 feature (P1814R0).

[HowardHinnant]

Prefer to use hh_mm_ss in place of time_of_day. time_of_day got renamed to hh_mm_ss during the standardization process for C++20, and so time_of_day remains strictly as a backwards compatible shim for current and past users of this lib.