Rename `listeners.stackable.tech/listener-class` PVC annotation to `listeners.stackable.tech/class`

[nightkr]

It's inconsistent with secret-op (which has secrets.stackable.tech/class).

We should keep supporting the old annotation key for compatibility.

[nightkr]

Actually.. this would be difficult for operators that already use the old key for persistent listeners to change. Maybe it's better to at least be internally consistent within the operator...