Update springboot.config with correct properties

[WadeWaldron]

Description

As per the documentation here:

https://docs.spring.io/spring-boot/docs/current/reference/html/application-properties.html#application-properties.integration.spring.kafka.bootstrap-servers

The correct configuration for the bootstrap servers is spring.kafka.bootstrap-servers rather than spring.kafka.properties.bootstrap.servers

You can see here where there is some discussion about the spring.kafka.properties.bootstrap.servers being supported unintentionally and having since been removed.

Author Validation

Describe the validation already done, or needs to be done, by the PR submitter. Please note submitters are expected to build docs-platform locally to validate before merging from examples.

I have validated this using the course materials found here: https://developer.confluent.io/courses/spring/apache-kafka-intro/

Reviewer Tasks

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