refactor: replace deprecated utcfromtimestamp function

[tdadela]

Replace deprecated datetime.datetime.utcfromtimestamp function with datetime.datetime.fromtimestamp.
See datetime.datetime.utcfromtimestamp docs:

Warning: Because naive datetime objects are treated by many datetime methods as local times, it is preferred to use aware datetimes to represent times in UTC. As such, the recommended way to create an object representing a specific timestamp in UTC is by calling datetime.fromtimestamp(timestamp, tz=timezone.utc).

Deprecated since version 3.12: Use datetime.fromtimestamp() with UTC instead.

It currently generates warning in Locust CI.