Bug 2046584 - Remove getPreviousGeckoPrefStates from public NimbusClient API (#7412)

## Nimbus

- Enrollment change events (visible to the UDL) now include the feature IDs of the features involved when possible. ([#7391](https://github.com/mozilla/application-services/pull/7391/))

- Removed NimbusInterface.getPreviousGeckoPrefsState since it is unimplemented and the underlying SDK method is only used by tests. ([#7412](https://github.com/mozilla/application-services/pull/7412/))

## 🔧 What's Fixed 🔧

### Logins

        geckoPrefStates: List<GeckoPrefState>,

    ) = Unit

    /**

     * Retrieves a list of previous states, if available, on an enrolled experiment, from a given slug.

     *

     * @param experimentSlug The slug of the experiment.

     * @return The previous Gecko pref states of the given slug. Will return null if not available or invalid slug.

     */

    fun getPreviousGeckoPrefStates(

        experimentSlug: String,

    ): List<PreviousGeckoPrefState>? = null

    /**

     *  Reset internal state in response to application-level telemetry reset.

     *  Consumers should call this method when the user resets the telemetry state of the