[BUG] [Spring] example response uses platform default charset

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 an issue with the ApiUtil class - reliance on default encoding

openapi-generator version

3.3.4

OpenAPI declaration file content or url

This is in ApiUtil, so I suspect it is present in any generated API server.

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

Generate the api (mvn compile)

Related issues/PRs

None that I can see

Suggest a fix

I think the issue is here: https://github.com/OpenAPITools/openapi-generator/blob/68a8f2976cd0bf7af67d7568bfbba84d689cc6b2/modules/openapi-generator/src/main/resources/JavaSpring/apiUtil.mustache#L30

I see UTF-8 used elsewhere, so I think the call to String.getBytes should specify the UTF-8 Charset.