Resolve datetime deprecation warnings

[Emmanuel Ferdman (emmanuel-ferdman)]

Description

This small PR resolves the datetime deprecation warnings which you can see in the CI logs:

/home/runner/work/cfripper/cfripper/cfripper/boto3_client.py:25: 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).
    now = datetime.utcnow().isoformat().replace(":", ".")

Checklist

• I have updated the CHANGELOG.md file accordingly

[Jordi Soucheiron (jsoucheiron)]

Thanks for the fix. We'll release it shortly