DEV: remove deprecations from Ember 3/4 after upgrade to Ember 5

[tyb-talks]

We've upgraded to Ember 5, so no need to track v3/4 deprecations.

Can be verified against https://github.com/ember-learn/deprecation-app/tree/main/content/ember/v5 (yes there's only 1).

[davidtaylorhq]

Hmm I think the test failure is probably because we've silenced deprecate-implicit-route-model in core's deprecation-workflow config 🤔. Perhaps we should update the system spec to unsilence everything before triggering deprecations? Something like

page.execute_script <<~JS
  window.deprecationWorkflow.config.workflow.clear()
JS

Then I think it should work consistently even when we change core 🤞