Move Kotlin code samples to a dedicated package

[sdeleuze]

The upgrade to Kotlin 2 planned in #33629 is blocked by an error that happens during the compilation of Kotlin code samples due to duplicated classes between Java and Kotlin code samples, since they are using the same package and name.

After discussing with @philwebb and @rwinch, we agreed to try to solve this by using a dedicated package per language, maybe with a related convention. This will likely require evolving the include-code extension where I will potentially create a related PR.

That change will also avoid warnings/errors in the IDE: