[Spike - API] Ensure all dates are UTC

Overview of the Feature Request
This spike was triggered by a discussion of how to present dates in the SPA. We would like all dates being returned from the API to be UTC. The UI can convert those dates to the user's local timezone. Part of what is to be researched is how the dates are stored in Postgres, because that will affect the logic of APIs that handle dates. The current assumption is that all dates are stored as UTC, but we want to confirm that.

What kind of user is the feature intended for?
(Example users roles: API User, Curator, Depositor, Guest, Superuser, Sysadmin)
API User

What inspired the request?
Logic for displaying dates in the SPA

What existing behavior do you want changed?
Want to ensure all dates are returned as UTC

Any brand new behavior do you want to add to Dataverse?

Any open or closed issues related to this feature request?

Start by auditing the API responses that return dates and confirming how those values are stored in PostgreSQL. Document which date fields are already UTC, identify exceptions, and define the API-wide behavior needed for the SPA to convert dates locally.