chore(test-app): import ember-data string transform

[BobrImperator]

• Fixes the following deprecation

DEPRECATION: You are relying on ember-data auto-magically installing the StringTransform. Use export { StringTransform as default } from '@ember-data/serializer/transform'; in app/transforms/string.js instead [deprecation id: ember-data:deprecate-legacy-imports] This will be removed in ember-data 6.0.

• Changes ember-lts-3.28 testing scenario to use ember-data LTS 4.4.0.
  ember-data 3.28 doesn't have the transform export.