Deprecation warnings with Ember 3.21

Ember 3.21 [deprecated getWithDefault](https://github.com/emberjs/ember.js/blob/master/CHANGELOG.md#v3210-august-24-2020). `ember-resize` uses it in a couple of places - [here](https://github.com/mike-north/ember-resize/blob/v0.3.4/app/initializers/resize.ts#L3) and [here](https://github.com/mike-north/ember-resize/blob/v0.3.4/addon/services/resize.ts#L1). This leads to a bunch of warnings.