[java-micronaut] make parent with a discriminator abstract and sealed (!12361) · Merge requests · OpenAPI Tools / openapi-generator

Created by: auke-

Parent types with a discriminator which aren't part of the mapped types should be abstract, since they can't be instantiated themself.

Furthermore an option is added which marks the parent class as sealed in order to let the Java compiler know which subtypes exist.

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```
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