Dropping Node 10, older versions of Ember

[nlfurniss]

Unless anyone objects, I'm going to update the expected Node version, plus drop old LTS versions of Ember. The old versions of Ember are blocking dependency bumps (ember-data, etc), converting the dummy app to Octane, etc.

Once that's done I'm going to spend some time getting tests running ember-canary (a.k.a v4) to pass.

cc @xg-wang @stefanpenner @rwjblue