Replace "weird" selector labels with something more conventional

[lann]

Early in the development of spin-operator I introduced this "weird" approach with service selector labels where the key is dynamic and contains the app ID. AFAIK nothing actually depends on this weird behavior today, so it would probably be good to switch to a more conventional selector. Pods are already generated with both the "weird" labels and a "normal" core.spinkube.dev/app-name: <app-id> label, so the service selector can probably just switch to the latter, followed by removing the former assuming it is unused.