Remove/replace Virtus models for EVSS - Part 1

[stevenjcumming]

Summary

• Replace Virtus model with Vets::Model
• Boolean changes to Bool (this avoids a naming conflict)
• The other Virtus models will be updated in another PR
• Some models where treated like hashes so they needed to be updated
  • For example rated_disability['decision_code'] vs rated_disability.decision_code
  • 2 Mobile legacy models needed that used EVSS models to be updated as well
• Vets::Model related updates addressed in a different PR that should be merged first
  • https://github.com/department-of-veterans-affairs/vets-api/pull/18691

Related issue(s)

• https://github.com/department-of-veterans-affairs/va.gov-team/issues/92632

Testing done

• [ ] manually testing to verify objects are the same with virtus and Vets::Model

Acceptance criteria

• [ ] model(s) don't use the virtus gem