minor, but easy fix: replace deprecated function datetime.utcfromtimestamp()

In the context of reviewing !77 (merged) I found out that there was a call to a function that's marked as deprecated in python 3.12: datetime.utcfromtimestamp().

Now, in the immediate we're planning to run the donate django site with python 3.11 so it's not affecting us, but since we know that it's deprecated and has a replacement that can already be used, we should swap the call to the recommended alternative, datetime.fromtimestamp(thething, datetime.UTC)

see: https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp

/cc @stephen