Python 3.12: datetime.utcnow() is deprecated

With  #7016, starting to test on different Python versions, 3.12 fails on a deprecation warning which is out of the scope of that PR:

```
ERROR tests/test_app.py::test_notebook_handler - DeprecationWarning: datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.now(datetime.UTC).
```
