Add Kotlin version for spring-boot-to-micronaut-at-singleton-at-component

[sdelamo]

The guide guides/spring-boot-to-micronaut-at-singleton-at-component is missing a Kotlin version.

Expected work:

• Add the Kotlin sample implementation for the guide.
• For multi-application guides, add the Kotlin implementation under each application directory that needs it.
• Update guides/spring-boot-to-micronaut-at-singleton-at-component/metadata.json to include KOTLIN in languages.
• Run ./gradlew springBootToMicronautAtSingletonAtComponentBuild --stacktrace.
• Run ./gradlew springBootToMicronautAtSingletonAtComponentRunTestScript --stacktrace.

This issue was created from an audit of guides missing Java, Groovy, or Kotlin variants. Groovy variants are intentionally not requested for Graal/GraalVM-specific guides.