Deprecate ajax for fetch

[snewcomer]

Right now, ember-data's adapters connote the use of jquery/ajax through public-ish methods.

See minimum-adapter-interface for the proper public API of ember-data's adapters. Note there is no mention of ajax, but there are many methods that users tend to override in their adapters.

With Ember 4.0 dropping jQuery, we should take a hard stance on dropping the use of Ajax in the next major release. Currently in 4.0, ember-data does suggest the use of fetch in their adapters but can be overridden by setting useFetch = false.

In managing our transition to fetch based methods, we can do some light maintenance work to expose fetch related methods and drop switching between fetch and ajax with useFetch. The goal here would be to help improve user's understanding and use of our adapters. Moreover, this refactor may help us take a very small step to a world where we completely rethink the idea of adapters/serializers.

https://github.com/ember-cli/ember-fetch

[wagenet]

Can we just make the RFC for this?

[runspired]

@wagenet there is #790