Fix: Ensure consistent datetime handling during CSV import

[jkppr]

This PR addresses a bug where the CSV importer would fail with a TypeError when encountering certain datetime formats with timezone offsets, even if those formats were individually parseable by dateutil. The error stemmed from Pandas' inconsistent timezone inference across chunks of the DataFrame.

The fix adds utc=True to the pandas.to_datetime call within the read_and_validate_csv function in timesketch/lib/utils.py. This forces all parsed datetimes to be explicitly represented in UTC, preventing timezone-related parsing errors and ensuring consistent datetime handling.

Key Benefits:

• Improves data consistency by storing all datetimes as UTC.
• Prevents TypeError during CSV import for a broader range of datetime formats.

This change is backward compatible and should not affect existing timelines or functionality.