[kotlin-spring] Generated code using Schema with discriminator mapping will not compile

[Philzen]

Closes #14949

Currently the CI jobs for all my other PRs are failing b/c i activated testing of the kotlin-springboot-request-cookie as part of #18809 (one would assume that should teach me to stay strictly in scope in my PRs 🙈).

I'm a bit concerned why that did not come up in that PRs CI runs… we should investigate. However, on the positive side it made me get around to fix #14949.

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[Philzen]

@wing328 Kindly review this before any other PRs – apparently all other merges will be blocked by it.