fix: dev: Change the NS_PER_SEC (and friends) from enum to static const

New version of clang (19) has introduced a stricter checks when mixing
integer (and float types) with enums.  In this case, we used enum {}
as C17 doesn't have constexpr yet.  Change the time conversion constants
to be static const unsigned int instead of enum values.

Closes #4845

Merge branch '4845-change-NS_PER_SEC-type-from-enum-to-integer' into 'main'

Closes #4845

See merge request isc-projects/bind9!9313