[BUG][Java] ServerConfiguration is not included in imports when generating from simple OpenAPI file

[cdelannoy-leroymerlin]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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 (example)

Description

When upgrading OpenAPI, I noticed this PR introduced ServerConfiguration for the restclient library but the generated code does not compile because the ServerConfiguration and ServerVariable POJOs are not imported.

openapi-generator version

7.15.x and 7.16.x

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Metric ACL API
  description: Metric ACL API for managing access control lists.
  version: 3.3.0
paths:
  # paths
components:
  securitySchemes:
    # security scheme definitions
  parameters:
    # parameter definitions
  headers:
    # header definitions
  schemas:
    # schema definitions

Generation Details

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>${openapi-generator.version}</version>
                <executions>
                    <execution>
                        <id>generate-java-http-client</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <output>${openapi.generation.directory}</output>
                            <inputSpec>${project.basedir}/src/main/resources/api-characteristic.yaml</inputSpec>
                            <generatorName>java</generatorName>
                            <generateApiTests>false</generateApiTests>
                            <generateModelTests>false</generateModelTests>
                            <generateApiDocumentation>false</generateApiDocumentation>
                            <generateModelDocumentation>false</generateModelDocumentation>
                            <generateSupportingFiles>true</generateSupportingFiles>
                            <supportingFilesToGenerate>ApiClient.java,Authentication.java,HttpBasicAuth.java,HttpBearerAuth.java,ApiKeyAuth.java,JavaTimeFormatter.java,RFC3339DateFormat.java</supportingFilesToGenerate>
                            <configOptions>
                                <sourceFolder>.</sourceFolder>
                                <groupId>${project.groupId}</groupId>
                                <invokerPackage>${openapi.generation.external.admintool.base.package}</invokerPackage>
                                <apiPackage>${openapi.generation.external.admintool.base.package}.api</apiPackage>
                                <modelPackage>${openapi.generation.external.admintool.base.package}.model</modelPackage>
                                <generateClientAsBean>true</generateClientAsBean>
                                <dateLibrary>java8</dateLibrary>
                                <artifactId>${openapi.generation.artifact.id.package}</artifactId>
                                <artifactVersion>${project.version}</artifactVersion>
                                <configPackage>${openapi.generation.external.admintool.base.package}.configuration</configPackage>
                                <basePackage>${openapi.generation.external.admintool.base.package}</basePackage>
                                <library>restclient</library>
                            </configOptions>
                            <typeMappings>
                                <typeMapping>OffsetDateTime=LocalDateTime</typeMapping>
                            </typeMappings>
                            <importMappings>
                                <importMapping>java.time.OffsetDateTime=java.time.LocalDateTime</importMapping>
                            </importMappings>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

• Run mvn install using provided OpenAPI spec file and Maven plugin config

Related issues/PRs

• [Java][RestClient] Generate servers for restclient library #21699

Suggest a fix

Import the ServerConfiguration and ServerVariable imports in the mustache file.