Expand support back to Ember 2.12

[mixonic]

Unlike many other addons, ember-cli-deprecation-workflow is often used with older Ember versions as it provides the upgrade path. This patch expands the supported versions on master (which will become 2.0, see https://github.com/mixonic/ember-cli-deprecation-workflow/issues/109) back to Ember 2.12.

Notable:

• This lowers the version of test helpers used so that they no longer rely on the destroyable polyfill (not backwards compatible to Ember 2.x)
• However lowering that version will make it difficult or impossible to adopt new changes in that addon to avoid touching the Ember global. The right approach may be to use a different test of test helpers based on the Ember version via ember-try as all this stuff is about the test suite, not the actual addon.
• Additionally the older version of test helpers is not compatible with Node 16.

[mixonic]

I'm going to merge this as it gets our suite green for a thorough list of Ember versions. I will flag the issues around test helpers and Node versions at https://github.com/mixonic/ember-cli-deprecation-workflow/issues/109