[kotlin-server] fix bean validation site targets

[stephan-strate]

Changed the site target of jax-rs specific annotations for generated models to @field. That allows the validator to see these annotations and validate accordingly.

fixes #12351

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[stephan-strate]

fyi: @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

[stephan-strate]

Any opinion on that?

[gdiegel]

@jimschubert Could this be merged? I'm currently struggling with bean validation not working with the kotlin-server generator and this PR would fix it.