WIP: Getting ready for Ember 4

[Baltazore]

I tried to do #690 twice last week, both times I failed due to crashes in test-app and later in docs-app

I noticed that for my app I mostly needed this add-on to be up to date with its internal dependencies, but also wanted to at least update docs ( they are few versions behind now )

This is still WIP, but I want to show progress, and get feedback sooner.

TODO:

• Fix Deprecation for service:fastboot

DEPRECATION: As of Ember 4.0.0, owner.inject no longer injects values into resolved instances, and calling the method has been deprecated. Since this method no longer does anything, it is fully safe to remove this injection. As an alternative to this API, you can refactor to explicitly inject `_fastbootInfo` on `service:fastboot`, or look it up directly using the `getOwner` API. [deprecation id: remove-owner-inject] See https://deprecations.emberjs.com/v4.x#toc_implicit-injections for more details.

• Fix assert.expectAssertion is not a function
  From what I see the addon is not maintained anymore, not sure what to do with it