Use TIMESTAMP_TRUNC instead of DATETIME_TRUNC for BigQuery

[courtneyholcomb]

A user flagged that the DATETIME_TRUNC operation was causing a query to not use their BQ date partition properly, resulting in more GB of data processed than expected.
After doing some research, it looks like the DATETIME_TRUNC also converts a timestamp object to a datetime object, which removes any timezone info. We don't intend to ever change the timezone of the underlying column, so this is not what we want.
The downside of this change is that, for any BQ users who are using timezone-aware data that's not UTC, they might see a change in the output. This is because DATETIME_TRUNC converts to UTC by default before truncating. Regardless, most users using BigQuery should be using UTC as best practice.

This PR updates the BQ SQL renderer to use TIMESTAMP_TRUNC instead of DATETIME_TRUNC.