Fix ember-assign deprecation warning

[drouhard]

After updating to Ember 4 I am seeing thousands of these deprecation warnings coming from ember-data-factory-guy initializing.

https://github.com/emberjs/rfcs/blob/master/text/0750-deprecate-ember-assign.md

Fortunately the recommended solution is backwards-compatible and simple. Replace this:

import { assign } from '@ember/polyfills';

with this:

import { assign as emberAssign } from '@ember/polyfills';

const assign = Object.assign || emberAssign;