[BUG][CSHARP-NETCORE] Type boolean with enum constraint produces incorrect code.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request

Description

In an API, that constrains a boolean to true (via enum), it will produce invalid code when using the csharp-dotnet generator.

openapi-generator version

openapi-generator-cli-5.4.0-20220124.094022-41.jar
openapi-generator-cli-6.0.0-20220104.102705-29.jar

OpenAPI declaration file content or url

 CurrencyUpdate:
      type: object
      properties:
        default:
          type: boolean
          format: boolean
          example: true
          enum:
            - true

Generation Details

Output:

[JsonConverter(typeof(StringEnumConverter))]
public enum DefaultEnum
{
    /// <summary>
    /// Enum True for value: true
    /// </summary>
    True = true   //error - invalid
}

Steps to reproduce

java -jar openapi-generator-cli-5.4.0-20220124.094022-41.jar generate -i https://api-docs.firefly-iii.org/firefly-iii-1.5.4.yaml -g csharp-netcore -o ./output/

Suggest a fix

Start by running the csharp-netcore generator with the supplied OpenAPI declaration and reproduction command, then trace how the boolean enum is represented in the generated C# output. Done means the constrained boolean produces valid C# without the invalid enum assignment and the original reproduction no longer fails.