[BUG][ALL GENERATORS] enums with x-enum-varnames do not respect enumPropertyNaming

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? YES
• Have you validated the input using an OpenAPI validator (example)? YES
• What's the version of OpenAPI Generator used? 4.1.2
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

If an API is relying on x-enum-varnames to provide enums' names, enumPropertyNaming is not applied.

This is because the DefaultGenerator does not use toEnumVarName to process x-enum-varnames extensions.

openapi-generator version

4.1.2 (still present in master)

OpenAPI declaration file content or url

openapi: 3.0.1
info:
    title: 'ping test'
    version: '1.0'
servers:
    - url: 'http://localhost:9999/'
paths:
    /ping:
        get:
            operationId: pingGet
            responses: {'201': {description: OK}}
components:
    schemas:
        WeatherType: null
            type: integer
            format: int32
            enum:
                - 42
                - 18
                - 56
            x-enum-varnames:
                - Sunny
                - Cloudy
                - Rainy

Command line used for generation

generate -i api.yaml -g kotlin -o output -p enumPropertyNaming=UPPERCASE

Steps to reproduce

Generate, open WeatherType.kt

Expected output:

enum class WeatherType(val rawValue: kotlin.Int) {
    SUNNY(42),
    CLOUDY(18),
    RAINY(56);
}

Actual output:

enum class WeatherType(val rawValue: kotlin.Int) {
    Sunny(42),
    Cloudy(18),
    Rainy(56);
}

Related issues/PRs

None

Suggest a fix

When processing x-enum-varnames in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java#L4346-L4361 We should make sure to call toEnumVarName to correctly generate the name.
Another solution could be to delay to call to toEnumVarName until after x-enum-varnames has been processed in postProcessModelsEnum (https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java#L372-L378)

Start by reproducing the issue with the Kotlin generator and enumPropertyNaming=UPPERCASE, then inspect DefaultCodegen.java around lines 4346-4361 and postProcessModelsEnum around lines 372-378. Compare the generated WeatherType.kt names with the expected uppercase output; done means x-enum-varnames respects enumPropertyNaming.