Update all instances of utcnow()

[cnfait]

utcnow() is deprecated since Python 3.12: https://blog.miguelgrinberg.com/post/it-s-time-for-a-change-datetime-utcnow-is-now-deprecated

Since we're planning to upgrade everything to use this version (see #268), let's avoid using this method, as at the very least warnings could clutter logs.