'Last updated field' should not allow expiration and warning fields.

Problem/Motivation

The 'last updated field' on the Expirable Content Type edit form allows for circular logic.

Steps to reproduce

• Configure an Expirable Content Type for a node or another entity. The details of the configuration don't matter.
• Clear the Drupal cache.
• Edit the Expirable content type
• Under 'Last updated field', see that the 'expiration_date' and 'warning_date' fields are selectable.

Proposed resolution

Prevent setting this at the form level.
Consider adding a constraint.