PostgreSQL timestamps are always datetimes

The current behavior is that they are treated as `datetime` normally,
but if they are part of an array, they are treated as `timestamp`. The
only place that seems to be impacted by this is schema dumping, which
shouldn't matter since `t.datetime` and `t.timestamp` are equivalent in
the `PostgreSQL` adapter, anyway.
Merge pull request #15156 from sgrif/sg-postgres-timestamps

PostgreSQL timestamps should always be datetimes