[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 :see_no_evil:).

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.

PR checklist

• [ ] Read the contribution guidelines.
• [ ] Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• [ ] Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh

  (For Windows users, please run the script in Git BASH) Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.

• [ ] File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• [ ] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[Philzen]

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