Change type of artifactPath to String

[jmini]

The artifact_path is from type String:
https://docs.gitlab.com/ee/api/job_artifacts.html#download-a-single-artifact-file-by-job-id

Using a java.nio.file.Path to model this has very little benefit, since we have no real file system in that case.

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The current work-around is to create a ArtifactsFile to set the filename attribute as String:

ArtifactsFile file = new ArtifactsFile();
file.setFilename(artifactPath); 
InputStream inputisStream = gitLabApi.getJobApi().downloadArtifactsFile(idOrPath(project), jobId, file);

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IMO this breaking change on the 6.x branch is OK.