refactor: remove non strict and non explicit relationships deprecation

[jaredgalanis]

This one involves the following deprecations:

https://deprecations.emberjs.com/ember-data/v4.x#toc_ember-data-deprecate-non-strict-relationships

This one we already got for free by upgrading ember simple auth in the process of the auth refactoring:

ember-simple-auth.initializer.setup-session-restoration