DEP: Remove dependency on slf4j from picocli-spring-boot-starter

[remkop]

Use java.util.logging instead of slf4j library for logging in picocli-spring-boot-starter.

TBD: This allows us to replace requires org.slf4j in module-info with requires java.util.logging.
The idea is to only require modules spring.boot and spring.boot.autoconfigure instead of spring.boot.starter in our module-info.

(Note: ✏️ if spring.boot.starter is needed on the module-path when generating modular javadoc anyway, then there is no point in doing this and this idea can be dropped. 🤷 )

Applications that want to use slf4j can simply add dependency org.slf4j:jul-to-slf4j.

Applications that have a dependency on spring-boot-starter, or otherwise have a dependency on spring-boot-starter-logging already include dependency org.slf4j:jul-to-slf4j, so they should have no impact.