[BUG] Spring responseWrapper with java8 generate invalid default

Created by: Zomzog

Bug Report Checklist

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

When using responseWrapper the default interface implementation return ResponseEntity and not responseWrapper object.

openapi-generator version

4.0.0-SNAPSHOT

OpenAPI declaration file content or url

petstore

Command line used for generation

<configuration>
    <inputSpec>${project.basedir}/src/main/resources/api.yml</inputSpec>
    <generatorName>spring</generatorName>
    <library>spring-boot</library>
    <configOptions>
        <sourceFolder>src/gen/java/main</sourceFolder>
        <responseWrapper>ListenableFuture</responseWrapper>
        <java8>true</java8>
    </configOptions>
</configuration>

Steps to reproduce

Related issues/PRs

Suggest a fix

Disable jdk8 when using responseWrapper