Switch to using `timestamp with time zone` by default

https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_timestamp_.28without_time_zone.29

While all of the Ash built-in primitives work fine on top of timestamp without timezone, we want to push users towards best practices. To this end, we will (at some point) add an opt-in toggle that switches `AshPostgres` to use `timestamp with time zone` for all of its datetime storage. This should (barring any investigation done when it's time to do the work) essentially just transparently make things better for users. The migration generator would pick up any changes and migrate them to UTC timestamps with time zone, and then basically this just have less trouble when working directly with the database in the future (in fragments or in SQL queries)