[BUG][Java][Spring] Delegate interface has to have default methods only

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?
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Description

I'm using the open api plugin to generate spring stubs/models with delegate interface without default methods. Since version 6.0.0 I'm forced to generate delegate with default methods only. I'd like to have a delegate without default methods so further adjustments (like adding new methods) should throw compile errors and the developer should be forced to implement the new method.
If this is a feature and not a bug the flag skipDefaultInterface does not make much sense to me.

The generated controller:

@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", date = "2022-06-09T16:07:19.513564+02:00[Europe/Berlin]")
@Controller
@RequestMapping("${openapi.sample.base-path:/v1}")
public class UsersApiController implements UsersApi {

    private final UsersApiDelegate delegate;

    public UsersApiController(@Autowired(required = false) UsersApiDelegate delegate) {
        this.delegate = Optional.ofNullable(delegate).orElse(new UsersApiDelegate() {});
    }
}

Since UsersApiDelegate is an interface with abstract methods only the generated UsersApiController throws a compile error.
Because of this line

this.delegate = Optional.ofNullable(delegate).orElse(new UsersApiDelegate() {});

The instantiation of interface new UsersApiDelegate() {} will fail since the method implementations are missing.

openapi-generator version

6.0.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Sample API
  description: Optional multiline or single-line description
  version: 0.1.9
servers:
  - url: http://api.example.com/v1
    description: Optional server description, e.g. Main (production) server
  - url: http://staging-api.example.com
    description: Optional server description, e.g. Internal staging server for testing
paths:
  /users:
    get:
      summary: Returns a list of users.
      description: Optional extended description in CommonMark or HTML.
      responses:
        '200':    # status code
          description: A JSON array of user names
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string

Generation Details

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>6.0.0</version>
                <executions>
                    <execution>
                        <id>generate-service-api</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/api/service/v1.yml</inputSpec>
                            <output>${project.build.directory}/generated-sources</output>
                            <skipIfSpecIsUnchanged>true</skipIfSpecIsUnchanged>
                            <generatorName>spring</generatorName>
                            <apiPackage>sample.api.service.v1</apiPackage>
                            <modelPackage>sample.api.model.v1</modelPackage>
                            <configOptions>
                                <sourceFolder>openapi</sourceFolder>
                                <delegatePattern>true</delegatePattern>
                                <skipDefaultInterface>true</skipDefaultInterface>
                                <java8>false</java8> <!-- avoid build interface default implemenations -->
                                <dateLibrary>java8</dateLibrary> <!-- use OffsetDateTime -->
                                <enablePostProcessFile>true</enablePostProcessFile>
                                <interfaceOnly>false</interfaceOnly>
                            </configOptions>
                            <addCompileSourceRoot>true</addCompileSourceRoot>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

Please check prior steps.

Related issues/PRs

Suggest a fix

This is the commit containing controller changes for spring
https://github.com/OpenAPITools/openapi-generator/commit/380aaa55a187b90c800e7d6c0ac44d43e5d1cf14

Start with the Spring generator changes referenced in commit 380aaa55a187b90c800e7d6c0ac44d43e5d1cf14 and reproduce using the supplied OpenAPI YAML and Maven configuration. Check how skipDefaultInterface=true and java8=false affect the generated UsersApiController and UsersApiDelegate. Done means the generated controller compiles when the delegate interface has abstract methods, with regression coverage for this configuration.