Adding an Ember 3.28 LTS to the test matrix. We used to test is as "release" but now that has moved on.

And in order to keep the suite the same size, I'm dropping the scenario for 3.20. We still test 3.16, our oldest supported LTS, which should give us plenty of coverage. Things that work in 3.16 and 3.24 are very unlikey to be broken in 3.20.

Co-authored-by: Sam Van Campenhout <[email protected]>

This closes #1112 by removing our OneShot optimization hack in favor of turning on broccoli memoization by default.

Broccoli memoization should absolutely be broccoli's default. It's not yet, but we can turn the flag on here unless users have explicitly turned it off.