Replace Virtus with Vets::Model on AVS

[stevenjcumming]

Summary

• Virtus is discontinued and needs to be removed.
• Replace Virtus models with Vets::Model
• Vets::Model needed a minor update to handle the base case and AVS case
• Another minor update to the spec was required to make the keys symbols

Related issue(s)

• https://github.com/department-of-veterans-affairs/va.gov-team/issues/92443

Testing done

• [x] manually comparing the previous and new AfterVisitSummary object

Acceptance criteria

• [x] virtus models are now subclasses of Vets::Model
• [x] model(s) don't use the virtus gem