[BUG][APEX] Enum values can conflict with keywords as Apex is case-insensitive

Bug Report Checklist

• [YES ] 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?

• [Yes ] Have you search for related issues/PRs?

• [ YES] What's the actual output vs expected output?

• [Optional] Bounty to sponsor the fix (example)

Description

take example of a API specifications for CUFX 4.1.
A part of this specs is as below:

"billStatus": {
         "description": " [AHR] omit ",
         "enum": [
           "Due",
           "OverDue",
           "Cancelled",
           "Suspended",
           "Paid",
           "Void",
           "Dispute",
           "PaymentFailed"
         ],
         "type": "string"

The value 'Void' will conflict with keyword 'void'. The generated output is as below:

    public enum BillStatusEnum {
        DUE,
        OVERDUE,
        CANCELLED,
        SUSPENDED,
        PAID,
        VOID,
        DISPUTE,
        PAYMENTFAILED
    }

This issue is continuing from Swagger 2.0 as I tried with that first and then now with OAS3.0.

Also, the maximum enums one can define is 100. This creates the problem in the enums for say Countries.

openapi-generator version

OAS 3.0 (Git Clone at 25d39c36340b570a8243a3f9f36bd93aec653f06)

OpenAPI declaration file content or url

I am trying to generate client coe for below .
https://app.swaggerhub.com/apis/dlacroix01/CUFX/4.1

Command line used for generation

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i ~/Downloads/swagger-client-generated-2/swagger.json -g apex -o ./ClientsApex/CUFX4.1/

Steps to reproduce

After code is generated, try to deploy the code on salesforce org.

Related issues/PRs

Suggest a fix

There can be multiple approaches.

1. I created class which has constant strings for the enum. The problem is with JSON serialize/deserialize.
2. using globalpicklist/valueset as these will be useful. However the type-checking cant be performed, as we wouldnt be saving any objects.
   Also there are governor limits [https://help.salesforce.com/articleView?id=fields_creating_global_picklists.htm&type=5]
   We may have to brainstorm here.

Start with the linked CUFX 4.1 OpenAPI specification and reproduce generation using the listed openapi-generator-cli command with the Apex generator. Inspect the generated BillStatusEnum and test deployment to a Salesforce org, including JSON serialization and deserialization. Done means enum values such as Void no longer conflict with Apex keywords while preserving the original API values; the issue also raises a separate limit concern about enums with more than 100 values.