Configure Kotlin Serialization converter with dedicated method

[bclozel]

As of spring-projects/spring-framework#35733, Spring Framework has a dedicated method for configuring a Kotlin Serialization converter specifically.

We should reflect that in our DefaultClientHttpMessageConvertersCustomizer to use this new method.

[bcmedeiros]

I was just about to ask what would be the impact in Boot after the Framework change, and I saw this issue.

if an application has Kotlin Serialization on the classpath, it is expected to use a dedicated converter for serializing @serializable types.

Is the newly introduced starter still going to be needed after this ticket is addressed? I understood just having the library in the classpath was not enough, but after this comment in the framework issue, I'm a bit unsure about the starter actually doing anything.

[bclozel]

@bcmedeiros yes this change is still required and the dedicated module is important: this is what allows you to use configuration properties to drive the creation of the Json instance and the Kotlin Serialization converter itself.