[Transform] Use monotonic time in TransformScheduler

[przemekwitek]

System clock may not be monotonic, i.e.: it is possible, that the following snippet fails:

  long time1 = clock.millis();
  long time2 = clock.millis();
  assertThat(time2, is(greaterThanOrEqualTo(time1));

In the context of transforms, it can happen that after _schedule_now is called, the transform will not be processed instantly (as is the premise of _schedule_now), because the current time fetched in line (*) will be earlier than the transform's next scheduled time (which was set just a while ago):

        long currentTimeMillis = clock.millis();  // (*)

        // Check if the task is eligible for processing
        if (currentTimeMillis < scheduledTask.getNextScheduledTimeMillis()) {
            // It is too early to process this task.
            // Consequently, it is too early to process other tasks because the tasks are sorted by their next scheduled time.
            // Try again later.
            return false;
        }

This PR fixes it by ensuring that the next time is at least as big as the previous time.

Fixes #95445

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

Hi @przemekwitek, I've created a changelog YAML for you.

[droberts195]

LGTM