## 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!

posthog/tasks/process_event.py
ip: str,
    site_url: str,
    team_id: int,
    event: str,
    distinct_id: str,
    properties: Dict,
    timestamp: Union[datetime.datetime, str],
) -> None:
    elements = properties.get("$elements")
    elements_list = None
    if elements:
        del properties["$elements"]
        elements_list = [
            Element(
                text=el["$el_text"][0:400] if el.get("$el_text") else None,
                tag_name=el["tag_name"],
                href=el["attr__href"][0:2048] if el.get("attr__href") else None,
                attr_class=el["attr__class"].split(" ") if el.get("attr__class") else None,