21188 added support and auto-detection for Kotlinx Serialization.

As seen in https://github.com/spring-projects/spring-boot/issues/24238, this is causing too much issues when Spring applications are consuming kotlin libraries bringing this dependency transitively. This converter shadows Jackson for Java enums, even after changes made in #26147.

This means we should consider this converter like Gson and Jsonb: a fallback replacement if Jackson is not present. Spring users can still add this converter to the list; we just won't make the mere presence of this library on the classpath as a strong enough signal. Spring Boot users can use HttpMessageConverters to add/alter the list of converters.

Because this is a breaking change, I'm scheduling this for 7.0.