[java] allow to use setArtifactVersion() programmatically

Created by: jmini

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• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Java: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04)

Description of the PR

Currently it is not possible to use setArtifactVersion(String) when you use OpenAPI-Generator programmatically:

JavaJAXRSSpecServerCodegen config = new JavaJAXRSSpecServerCodegen();
config.additionalProperties()
            .put(JavaJAXRSSpecServerCodegen.USE_SWAGGER_ANNOTATIONS, "false");
config.setHideGenerationTimestamp(true);
config.setOutputDir(outputDirPath.toFile().getCanonicalPath());
config.setArtifactVersion("0.9.0-SNAPSHOT");

Prior to this PR, the code above had no effect, because the value was overridden.