Drop support for ember less than 3.28

[leoeuclids]

Why?

V2 addons only support ember >= 3.28, so the support for anything below that should be dropped. This PR is separate to from #508 to be featured on the 5.0.0 release changelog.

What it achieves

Allows the conversion into a V2 addon (In progress #508).

What this PR does

Changes the ember-try scenarios, dropping ember-lts-2.4, as well as adding ember-lts-4.4 and ember-lts-4.8. Updates the Readme compatibility table with the 5.0.0 version.

• [x] Remove ember-lts-3.24 scenario
• [x] Add ember-lts-4.4 scenario
• [x] Add ember-lts-4.8 scenario
• [x] Update Readme

[leoeuclids]

A11y tests should pass now.

[leoeuclids]

Third try's the charm.

[leoeuclids]

Thanks for merging!