[JAVA] Generated enums are not immutable

Description

Swagger-codegen version

Using version 2.3.1
Not a regression and does not seem to be changed in future versions.

Swagger declaration file content or url

definitions:
  productType:
    type: string
    enum: [Paid, Free]
    description: ProductType - Paid or Free

Command line used for generation

Using swagger-codegen-maven-plugin to generate the Java classes

Steps to reproduce

Using swagger-codegen-maven-plugin to generate the Java classes

Related issues/PRs

None found

Suggest a fix/enhancement

The variable value in the following generated code should be final.

public enum ProductType {
  
  PAID("Paid"),
  
  FREE("Free"),

  private String value;

  ProductType(String value) {
    this.value = value;
  }

  @JsonValue
  public String getValue() {
    return value;
  }

  @Override
  public String toString() {
    return String.valueOf(value);
  }

  @JsonCreator
  public static ProductType fromValue(String text) {
    for (ProductType b : ProductType.values()) {
      if (String.valueOf(b.value).equals(text)) {
        return b;
      }
    }
    return null;
  }
}

Locate the Java generation template used by swagger-codegen-maven-plugin for the enum shown in the issue, then compare its output with the provided ProductType example. Verify the generated enum field is final by regenerating the sample definition and inspecting the resulting Java class.