refactor(admin): replace deprecated datetime.utcnow() (#772)

Python 3.12 deprecated `datetime.utcnow()` in favour of the
timezone-aware `datetime.now(datetime.UTC)`.

Drop the `.replace(tzinfo=None)` that stripped the timezone from
`last_activity` to make it match the naive `utcnow()` result.
Both `last_activity` (parsed with `%z`) and the new
`datetime.now(datetime.UTC)` are now timezone-aware UTC values,
so the subtraction yields the same `timedelta` as before.

Author: tiborsimko

reana_server/reana_admin/cli.py

@@ -1049,8 +1049,8 @@ def interactive_session_cleanup(
 
         last_activity = datetime.datetime.strptime(
             session_status["last_activity"], "%Y-%m-%dT%H:%M:%S.%f%z"
-        ).replace(tzinfo=None)
-        duration = datetime.datetime.utcnow() - last_activity
+        )
+        duration = datetime.datetime.now(datetime.UTC) - last_activity
         if duration.days >= days:
             if dry_run:
                 click.echo(