text: Title: Deprecate `elapsed_time_to_string` accepting time as milliseconds
  Body: The `robot.utils.elapsed_time_to_string` utility function currently accepts the elapsed time to be formatted as an integer or float representing milliseconds. This made sense earlier, because the `elapsedtime` attribute of our result model objects contained milliseconds as well, but as part of #4258 we now use `elapsed_time` that contains a `timedelta`. `timedelta` has microsecond precision and also directly support seconds with their `total_seconds()` method. Our utils working with milliseconds doesn't thus make much sense anymore.
  
  The `elapsed_time_to_string` was already enhanced to accept the elapsed time as a `timedelta`, but I believe we should change it to consider ints/floats as seconds as well. Although it's unlikely this function is used outside our code base, that is certainly possible and just changing the behavior wouldn't be good. I thus believe we should deprecate the old behavior first. The problem is that the int/float value itself cannot tell should it be interpreted as seconds or milliseconds, so we need to add a new argument to control that. If the new argument is used, I believe it could be `seconds=True`, then the value is interpreted as seconds. If it's not used, we can interpret it as milliseconds and emit a deprecation warning.