Removal of availablePlugins breaks addons

A number of older addons break with `ember-cli-preprocess-registry` versions `4.0+` with the following error:

```
Cannot read properties of undefined (reading '...')
```

We've traced it down to the [removal](https://github.com/ember-cli/ember-cli-preprocess-registry/commit/22379e12919520a86101f0f8ef314b7de43e4352) of `availablePlugins` from `registry`.

We've seen this problem with the following plugins:
- [ember-basic-dropdown](https://github.com/cibernox/ember-basic-dropdown/issues/706)
- [ember-power-select](https://github.com/cibernox/ember-power-select)
- [ember-side-menu](https://github.com/tsubik/ember-side-menu)
- [and there seem to be more...](https://github.com/search?q=registry.availablePlugins+language%3AJavaScript&type=code&l=JavaScript)

Is this an intended breaking change?