[kotlin-spring] fix validation regression in kotlin-spring generator

[pstorch]

With introduction of the kotlin-misk generator in #20885, a regression #21238 was introduced in kotlin-spring generator.

This pull request reverts the changes in the kotlin-spring generator which were accidentally introduced by the above mentioned PR.

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[wing328]

@pstorch thanks for the PR

cc @andrewwilsonnew (the author of the kotlin misk server generator)

[wing328]

@pstorch what about adding a test in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/KotlinSpringServerCodegenTest.java to catch the issue moving forward?

something like https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/spring/SpringCodegenTest.java#L5493

[pstorch]

@wing328: good idea. Tests added.

[andrewwilsonnew]

LGTM, nice! Apologies