Upgrade this app to Ember 4

This app is on Ember 3. We should upgrade to Ember 4!

Instructions: https://guides.emberjs.com/release/upgrading/

Whoever picks up this issue could upgrade to v4.0 or a later version. It's up to you!

I am marking this good first issue, but it is only a good first issue for someone who has experience upgrading another Ember app.