Replaced deprecated datetime.utcnow() with datetime.now(datetime.UTC)

[gautamgiri-dev]

Fixes: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
datetime_now = datetime.datetime.utcnow()
warning that shows due to change in the behavior of datetime module in latest python

[kurtmckee]

These are not the only places that reference .utcnow(), and this change makes the datetime instances timezone-aware.

There have been multiple PRs to address this, but I think that #3239 is the only one that doesn't make the datetime instances timezone-aware.