SpringBoot PubSub Example House Cleaning

We should move the [spring boot pubsub example](https://github.com/dapr/java-sdk/tree/4e1fbbe2cebe6ca44d25ac733c72d0c137c429a7/spring-boot-examples) to `java-sdk/spring-boot-example/pubsub` then have the producer/consumer apps inside that folder. We should also have it purely be about a pubsub example. We should not mix workflows and pubsub in this example. [This will align with the java-sdk pattern of having examples in a folder, then showing per API examples in this manner](https://github.com/dapr/java-sdk/tree/4e1fbbe2cebe6ca44d25ac733c72d0c137c429a7/examples/src/main/java/io/dapr/examples)

Beginne in spring-boot-examples und vergleiche dessen aktuellen producer/consumer- und Workflow-Inhalt mit dem Beispielmuster unter examples/src/main/java/io/dapr/examples. Verschiebe das Spring Boot PubSub-Beispiel nach java-sdk/spring-boot-example/pubsub, belasse die producer- und consumer-Apps in diesem Ordner und entferne den Workflow-Inhalt, sodass es in dem Beispiel nur um PubSub geht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.