Change today_at_midnight default timezone to UTC
Summary: This made sense in the old dark days when schedules defaulted to system timezone, but now we are enlightened and use UTC as the default. Make the tests that rely on this function also use UTC.
Test Plan: BK
Reviewers: prha, alangenfeld, johann, sashank
Reviewed By: johann
Differential Revision: https://dagster.phacility.com/D7398