[8.x] Fixes model:prune --pretend command for models with SoftDeletes

[wgmv]

This PR should fix the model:prune --pretend command when used in combination with soft deleted models.
The withTrashed method is never called because the SoftDeletes class is not found in the array. Therefore the --pretend option always returns 0 when using a condition in the model like

 public function prunable()
    {
        return static::where('deleted_at', '<=', now()->subDays(30));
    }

the query will currently be appended with "where deleted_at is null"

[derekmd]

A test case should probably be added to tests/Database/PruneCommandTest.php to ensure this works as intended going forward?