add back support for ember 3.28

[mansona]

There is nothing implicit in this addon that is preventing support for Ember 3.28 so this PR widens the peerDependency on ember-source and adds 3.28 to the CI matrix

Edit: I'm not exactly sure why the ci.yaml diff is the whole file 🤔 if you turn on hide whitespace it shows the one line that I added so I'm assuming it's a change from tabs to spaces or something? your editor config file says it should be spaces so I assume my editor has updated them to what the config was expecting 👍

[jrjohnson]

Right now we're in alignment with the ember addon blueprint for 4.12.2. In fact we very much need to update to a newer version. Running that update is much harder with a customization like this in the mix. Given how very lightly maintained this addon is at the moment I don't know if that is a good idea.

[mansona]

So I tend to update things using the blueprint and keep support for older things until I need to drop them (because I add a feature that breaks a test in older Ember or something)

If you want I can open a PR that updates the blueprint right to the latest for you and keeps 3.28+ support? Is that something that you would like given what you said about these sorts of customisations?

[jrjohnson]

I'd merge that @mansona, let's go that route.