timers: rename validateTimerDuration to getTimerDuration

The function did not only validate the timer but it caused side
effects like a warning and potentially returned a different value
than the input value. Thus the name `validate` did not seem to be
appropriate.

PR-URL: https://github.com/nodejs/node/pull/26809


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>