[BUG][Spring] OpenAPI2SpringBoot inner class should be static

Created by: brabster

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

SpotBugs reports that this ExitException should be a static inner class.

openapi-generator version

3.3.4

OpenAPI declaration file content or url

Ir's in OpenAPI2SpringBoot, I think it'll be a small fix on this line: https://github.com/OpenAPITools/openapi-generator/blob/b5ede4b339b630259a48f71098e5d2941ccf40dd/modules/openapi-generator/src/main/resources/JavaSpring/libraries/spring-boot/openapi2SpringBoot.mustache#L44

Command line used for generation

Maven plugin:

                          <generatorName>spring</generatorName>
                          <configOptions>
                                <sourceFolder>src/gen/java/main</sourceFolder>
                                <java8>true</java8>
                                <reactive>true</reactive>
                                <apiPackage>foo.bar.api</apiPackage>
                                <modelPackage>foo,bar.model</modelPackage>
                                <delegatePattern>true</delegatePattern>
                            </configOptions>

Steps to reproduce

Suspect it'll be there in any Spring Boot API

Related issues/PRs

None that I can see.

Suggest a fix

I think it'll just be the keyword static. I'll check and pop a PR in when I get a sec.