kernel: reimplement k_uptime_get_32()

The current implementation does not return the low 32 bits of
k_uptime_get() as suggested by it's documentation; it returns the number
of milliseconds represented by the low 32-bits of the underlying system
clock.  The truncation before translation results in discontinuities at
every point where the system clock increments bit 33.

Reimplement it using the full-precision value, and update the
documentation to note that this variant has little value for
long-running applications.

Closes #18739.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>