Drop ember qunit less than v6

[NullVoxPopuli]

In preparation for making the v2 conversion flow, starting in #2027 easier, I'm extracting some of the more isolated changes.

For an overall plan for how v2 addon conversion works,

• I've written up a guide: https://gist.github.com/NullVoxPopuli/eafc7dad6547de5e730098498b829e1f
• high level conversion process: https://github.com/embroider-build/embroider/blob/main/docs/porting-addons-to-v2.md
• at the end, qunit-dom will be using the v2-addon blueprint https://github.com/embroider-build/addon-blueprint (which @mansona helps maintain 💪 )
• additional information for addon authors: https://github.com/embroider-build/embroider/blob/main/docs/addon-author-guide.md

Also, in part, to help with maintenance burden, supporting less is easier as well. ember-qunit is on v8 now, and v4 and v5 are very old and have "magic" build-time behavior, which can't be transferred to the v2 addon world.

Up next will be upgrading the embroider deps, because < v3 isn't really supported 🙃

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Outstanding questions for the maintainers:

• I've added a note to the README about installing older (aka, todays) qunit-dom, if folks have older ember-qunit -- with these changes it would still (probably) be possible to use with older ember-qunit, but all the automatic setup stuff would need to be handled by the consumer.

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Things to keep in mind: In my subsequent PRs to qunit-dom, there may be more breaking changes. I'll make sure each is called out in the PR that introduces them, if not extracted to a separate PR (which would be my preference, depending on the situation that calls for the change)

[NullVoxPopuli]

I also added the latest v4 LTS here, since we want some matrix in our matrix