replace addon testing ember 3.4 for 3.12

[kellyselden]

We are overdue for another bump in latest 2 ember LTS for addon testing support. This swaps 3.4 for 3.12.

[Turbo87]

@kellyselden why did you target release with this? seems like this will be a breaking change for most addons that adopt this change 🤔

[kellyselden]

We've always been shifting the last 2 LTS's in the blueprints. We actually should have done this in ember-cli 3.13's stable release when 3.12 became LTS, I just forgot. As with most our blueprint changes, addon's have a choice whether they want to track these changes and adjust their major version accordingly. Do you have concerns with this policy?

[Turbo87]

problem is just that if we release this in a patch version bump we have no way of communicating this change in the release blog post since we only do those for minor and major version releases

[kellyselden]

That's a good point. I forgot about release blog post.

[knownasilya]

Maybe do a blog post any way 🤷‍♂

[rwjblue]

Tweaking this to target beta (I don't think its great to land a breaking change patch release of 3.14) (basically reverting from release and reverting the revert from beta).