Add testing of Gradle 9

[melix]

While investigating #760 , I added testing of Gradle 9 to our build. This caused a few cascading issues:

1. we have to upgrade the Kotlin plugin version we use in tests to the latest 1.9.x branch
2. doing so breaks the KotlinApplicationWithTestsFunctionalTest with a class initialized at build time error

This error only happens when testing with GraalVM 17, not with 23. The workaround I have found is to add:

kotlin.annotation.AnnotationTarget
kotlin.annotation.AnnotationRetention

to the list of classes initialized at build time, but I'm not completely sure this is the right thing to do. /cc @sbrannen @marcphilipp

In any case it seems that the original issue doesn't occur outside of the context of Spring.