## Bug description
 
It looks like either timestamp or sent_at is being sent without timezone, causing [this issue](https://sentry.io/organizations/posthog/issues/1972106098/?project=1899813&query=is%3Aunresolved&statsPeriod=14d)

## Expected behavior

It should send both timestamps with timezone

## How to reproduce

1. 
2. 
3. 

## Environment

- PostHog cloud or self-managed?
- PostHog version/commit

## Additional context



#### *Thank you* for your bug report – we love squashing them!

ee/clickhouse/process_event.py
distinct_id: str,
    ip: str,
    site_url: str,
    data: dict,
    team_id: int,
    now: datetime.datetime,
    sent_at: Optional[datetime.datetime],
) -> None:
    data = {
        "distinct_id": distinct_id,
        "ip": ip,
        "site_url": site_url,
        "data": json.dumps(data),
        "team_id": team_id,
        "now": now.strftime("%Y-%m-%d %H:%M:%S.%f"),
        "sent_at": sent_at.strftime("%Y-%m-%d %H:%M:%S.%f") if sent_at else "",
    }
    p = KafkaProducer()
    p.produce(topic=KAFKA_EVENTS_WAL, data=data)