[Kotlin] enums should be uppercase

Description

In Kotlin enums should be uppercased, its not directly a rule in Kotlin to have enums in uppercase but all examples used in Kotlin documentation is uppercased.

See here https://kotlinlang.org/docs/reference/coding-conventions.html#property-names and https://kotlinlang.org/docs/reference/enum-classes.html

Swagger-codegen version

2.3.1

Steps to reproduce

I dupliacted integrationtests for scala in /swagger-codegen/modules/swagger-codegen/src/test/resources/integrationtests/scala and created kotlin test, and then I ran /swagger-codegen/modules/swagger-codegen/src/test/resources/integrationtests/kotlin/required-attributes.sh

Related issues/PRs

Related to #5769

Suggest a fix/enhancement

protected CodegenConstants.ENUM_PROPERTY_NAMING_TYPE enumPropertyNaming = CodegenConstants.ENUM_PROPERTY_NAMING_TYPE.camelCase;

in KotlinClientCodegen.java should be changed to

protected CodegenConstants.ENUM_PROPERTY_NAMING_TYPE enumPropertyNaming = CodegenConstants.ENUM_PROPERTY_NAMING_TYPE.UPPERCASE;

Start in KotlinClientCodegen.java and inspect the enumPropertyNaming setting. Run modules/swagger-codegen/src/test/resources/integrationtests/kotlin/required-attributes.sh and compare the generated enum names with the Kotlin conventions cited in the issue. Done means the Kotlin integration test produces uppercase enum names.