The new email format has eliminated preferences, so there is no way to set up auto delete in either Spam or Trash.
In the new format, on the Inbox page, click on the gear icon in the upper right corner near your name. Then click on Settings. The new format has no provision for setting deletion schedules though. Spam is stored for 7 days or 2,000 messages, whichever comes first
Trash is stored for 14 days or 5,000 messages, whichever comes first. You cannot change those settings.