Resolve datetime deprecation warnings

[emmanuel-ferdman]

PR Summary

This PR fixes the DeprecationWarning that appears in Python 3.12+ when using datetime.utcnow(). It replaces it with a more future-proof approach: datetime.now(timezone.utc).replace(tzinfo=None). This keeps the datetime object naive (without timezone info), which is important for parts of the code that expect it that way, but avoids the warning by using a timezone-aware method first:

# Before (deprecated)  
from datetime import datetime  
naive_time = datetime.utcnow()  # Raises warning  

# Common Fix (timezone-aware)  
from datetime import datetime, timezone  
aware_time = datetime.now(timezone.utc)  # Breaks naive-dependent code  

# This PR's Fix (naive but compliant)  
from datetime import datetime, timezone  
compliant_time = datetime.now(timezone.utc).replace(tzinfo=None)  # No warning, backward-compatible  

You can see this warning in the latest CI run:

  /home/runner/work/vizier/vizier/vizier/_src/service/vizier_service.py:716: 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).
    or datetime.datetime.utcnow()