Use explicit use‑site target annotations for @JsonProperty in generated Kotlin code

When generating Kotlin classes with Kotlin 2.2+, each @JsonProperty annotation currently produces the following compiler warning:

w: file:///C:/git/dgs-sample/build/generated/sources/dgs-codegen/com/netflix/dgs/codegen/generated/types/SampleType.kt:14:3 This annotation is currently applied to the value parameter only, but in the future it will also be applied to field.

• To opt in to applying to both value parameter and field, add '-Xannotation-default-target=param-property' to your compiler arguments.
• To keep applying to the value parameter only, use the '@param:' annotation target.

See https://youtrack.jetbrains.com/issue/KT-73255 for more details.

Additional context is available here:
https://blog.jetbrains.com/idea/2025/09/improved-annotation-handling-in-kotlin-2-2-less-boilerplate-fewer-surprises/

To avoid these warnings and ensure consistent behavior across Kotlin versions, the code generator should use explicit use‑site targets. Instead of:

@JsonProperty("fieldName")

the generator should emit:

@field:JsonProperty("fieldName")

This explicitly applies the annotation to the field and matches the recommended approach for Kotlin 2.2+.