Resolve datetime utcnow deprecation warnings

[emmanuel-ferdman]

PR Summary

This small PR resolves the deprecation warnings on datetime in Python3.12+. You can find them in the CI logs:

  /__w/khoj/khoj/src/khoj/processor/content/images/image_to_entries.py:61: 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).
    timestamp_now = datetime.utcnow().timestamp()

[debanjum]

Thanks for submitting a fix for the utcnow() deprecation warning!