TimingUtils.cmake: Replace execute_process(date ...) with string(TIMESTAMP) with microsecond support with CMake 3.23

[bartlettroscoe]

Blocked by:

• • Upgrade CMake to most current version possible for Trilinos 14.0 trilinos/Trilinos#10355

With CMake 3.23, the string(TIMESTAMP ...) command now supports microseconds %f:

%f
     The microsecond of the current second (000000-999999).

This would allow us to replace the expensive execute_process(date ...) command in timer_get_raw_seconds() with a much faster and more portable string(TIMESTAMP ...) command.