Document 7.5 Operator: Remove reference to scaledown CRD

Remove references to scaledown CRD.

In 7.4:

To enable message migration for v1alpha1 CRDs requires deploying a custom resource instance of the scaledown CRD with localOnly true. Nothing else is supported. This causes the now independent scaledown controller to be instantiated and run.

In 7.5:

Set the messageMigration to true in the main broker CRD. This instantiates a custom resource instance of the scaledown CRD with localOnly true. Users no longer need to explicitly deploy a scaledown CR. We need to remove explicit mentions of this CRD and the examples.