[Java] Issue with code generation

Description

Code is not generated properly.

Content.java file contains wrong class definitions:
@JsonSubTypes({
@JsonSubTypes.Type(value = textMessage.class, name = "text"),
@JsonSubTypes.Type(value = carouselMessage.class, name = "carousel"),
@JsonSubTypes.Type(value = fileMessage.class, name = "file"),
@JsonSubTypes.Type(value = formMessage.class, name = "form"),
@JsonSubTypes.Type(value = formResponseMessage.class, name = "formResponse"),
@JsonSubTypes.Type(value = imageMessage.class, name = "image"),
@JsonSubTypes.Type(value = listMessage.class, name = "list"),
@JsonSubTypes.Type(value = locationMessage.class, name = "location"),
@JsonSubTypes.Type(value = templateMessage.class, name = "template"),
})
they should be capital letter class names

Swagger-codegen version

3.0.25

Swagger declaration file content or url

OpenApi file: https://github.com/zendesk/sunshine-conversations-api-spec/blob/master/openapi.yaml

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i openapi.yaml -l "spring"

Steps to reproduce

just generate sources

Related issues/PRs

Suggest a fix/enhancement

Reproduce the generation using the linked OpenAPI file and the documented swagger-codegen 3.0.25 command with the spring generator. Inspect the generated Content.java definitions and the generator templates or entry points responsible for these names; done means the generated class names begin with capital letters.