Remove ember-notify-property-change-polyfill

[fsmanuel]

The readme states that version > 1.0.1 requires ember >= 3.8 and the polyfill is only used for emberVersion.lt('3.1.0'):
https://github.com/rondale-sc/ember-notify-property-change-polyfill/blob/master/index.js#L13

So I think it's save to remove the dependency. That also removes this deprecation:

[fsmanuel]

@lukemelia
Our tests fail for ember 3.8 because we use @action decorators:
https://github.com/yapplabs/ember-buffered-proxy/runs/6387142409?check_suite_focus=true

For ember 3.12 one test is failing because tracked properties were introduced in 3.13:
https://github.com/yapplabs/ember-buffered-proxy/runs/6387142489?check_suite_focus=true

I think everything should work for 3.12 if you use @computed decorators but the effort to cover it in the tests is to much. So I updated the readme to reflect the compatibility.

[lukemelia]

Released as 2.1.1