Replace deprecated `datetime.utcnow()` w/ a tz-aware variant

[radez]

Calling datetime.datetime.utcnow() emits a deprecation warning now. Python 3.12 reports:

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).

This patch replaces it with a non-deprecated cross-python implementation.

What kind of change does this PR introduce?

• [x] refactoring

Checklist:

• [x] I think the code is well written
• [x] I wrote good commit messages
• [x] I have squashed related commits together after the changes have been approved
• [x] I used the same coding conventions as the rest of the project
• [x] The new code doesn't generate linter offenses
• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

[webknjaz]

I have clicked the "Rebase" button via GH UI.