refs https://github.com/TryGhost/Ghost/issues/14101
- migrated component to Glimmer
- swapped usage of `ValidationState` mixin for `{{validation-status}}` modifier
- updated modifier to accept custom error/success classes
- removed unnecessary/unused code in the `gh-benefit-item.js` backing class
no issue
- moved logic from `<GhValidationStatusContainer>` to a new `validation-status` modifier
- removes a usage of the `ValidationState` mixin
- migrated uses of the component to a mixin
- paves the way for full removal of the `ValidationState` mixin in later refactors (mixins are deprecated)
- migrated `<GhFormGroup>` to a glimmer component
- swapped the extend of `GhValidationStatusContainer` to usage of the `validation-status` modifier with a template-only component
- updated all `<GhFormGroup>` to use the standard `class=""` instead of `@classNames=""` and `@class=""`
- allows `data-test-*` attributes to be added to uses of `<FormGroup>` to help when complex components are grouped as a form input
