Drop Node 11 from CI.
Drop Node 11 from CI.

* Does **not** changes "engines" in `package.json` (doesn't seem
terribly useful).
* Makes Node 12 the default for jobs that do not override
* Removes Node 11 CI job

buschtoens/ember-cli,thoov/ember-cli,kategengler/ember-cli,thoov/ember-cli,raycohen/ember-cli,elwayman02/ember-cli,ember-cli/ember-cli,ember-cli/ember-cli,jrjohnson/ember-cli,HeroicEric/ember-cli,thoov/ember-cli,kategengler/ember-cli,balinterdi/ember-cli,buschtoens/ember-cli,jgwhite/ember-cli,HeroicEric/ember-cli,HeroicEric/ember-cli,raycohen/ember-cli,HeroicEric/ember-cli,balinterdi/ember-cli,jgwhite/ember-cli,thoov/ember-cli,asakusuma/ember-cli,jgwhite/ember-cli,jgwhite/ember-cli,asakusuma/ember-cli,jrjohnson/ember-cli,ember-cli/ember-cli,elwayman02/ember-cli