[BUG] [Java] Generated files with option supportFilesToGenerate=false results in Compilation Errors

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?
• [Optional] Bounty to sponsor the fix (example)

Description

I have following Maven Plugin set up:

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
                            <generatorName>spring</generatorName>
                            <generateSupportingFiles>false</generateSupportingFiles>
                            <apiPackage>de.github.sparsick.openapi.spring.boot.contract.first.api</apiPackage>
                            <modelPackage>de.github.sparsick.openapi.spring.boot.contract.first.model</modelPackage>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

and the openapi.yaml is the pet store example

Then mvn clean compile results in

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project openapi-spring-boot-contract-first: Compilation failure: Compilation failure: 
[ERROR] /home/sparsick/dev/workspace_github/openapi-spring-boot/contract-first/target/generated-sources/openapi/src/main/java/de/github/sparsick/openapi/spring/boot/contract/first/api/PetsApi.java:[79,21] cannot find symbol
[ERROR]   symbol:   variable ApiUtil
[ERROR]   location: interface de.github.sparsick.openapi.spring.boot.contract.first.api.PetsApi
[ERROR] /home/sparsick/dev/workspace_github/openapi-spring-boot/contract-first/target/generated-sources/openapi/src/main/java/de/github/sparsick/openapi/spring/boot/contract/first/api/PetsApi.java:[108,21] cannot find symbol
[ERROR]   symbol:   variable ApiUtil
[ERROR]   location: interface de.github.sparsick.openapi.spring.boot.contract.first.api.PetsApi
[ERROR] -> [Help 1]

My expectation is that the generator is generating everything what I need to compile the API and the model.

Workaround:
It works when I change my Maven plugin configuration in

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
                            <generatorName>spring</generatorName>
                            <supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate>
                            <apiPackage>de.github.sparsick.openapi.spring.boot.contract.first.api</apiPackage>
                            <modelPackage>de.github.sparsick.openapi.spring.boot.contract.first.model</modelPackage>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

openapi-generator version

openapi-generator-maven-plugin in version 4.3.0

OpenAPI declaration file content or url

https://github.com/sparsick/openapi-spring-boot/blob/master/contract-first/src/main/resources/openapi.yaml

Command line used for generation

Using following Maven plugin configuration:

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
                            <generatorName>spring</generatorName>
                            <generateSupportingFiles>false</generateSupportingFiles>
                            <apiPackage>de.github.sparsick.openapi.spring.boot.contract.first.api</apiPackage>
                            <modelPackage>de.github.sparsick.openapi.spring.boot.contract.first.model</modelPackage>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

1. Checkout https://github.com/sparsick/openapi-spring-boot
2. cd contract-first
3. Delete in pom.xml line <supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate>
4. Uncomment in pom.xml line <generateSupportingFiles>false</generateSupportingFiles>
5. Run mvn compile

Related issues/PRs

Suggest a fix

ApiUtil.java should be a part of the must have files.

Start with the Maven plugin configuration in the issue and the spring generator output, focusing on the generated PetsApi.java references to ApiUtil.java. Reproduce the failure with the linked pet store OpenAPI declaration by running mvn compile after disabling generateSupportingFiles. Done means the generated API and model compile without requiring the supportingFilesToGenerate workaround.