[Kotlin] Fix (de)serialization of enum classes

[cschramm]

While inner enum classes from dataClass.mustache work fine, standalone enum classes lack @JsonProperty annotations so that Jackson uses names instead of values.

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

[wing328]

Can you please merge the latest master into your branch and update the samples by running the following?

./mvnw clean package 
./bin/generate-samples.sh

[cschramm]

Done. Those commands did not have any visible effect on the repository, though.

[wing328]

CircleCI tests passed via https://app.circleci.com/pipelines/github/OpenAPITools/openapi-generator/10330/workflows/ebfeac15-b026-4027-84eb-86452931e32d/jobs/23950