Resolve "Impossible to remove orphan artifact files"

What does this MR do?

Changes an invalid --class option passed to ionice to use -c instead.

According to the ionice man-pages ...

-c class The scheduling class. 0 for none, 1 for real time, 2 for best-effort, 3 for idle.

We already use the correct -c option elsewhere in our code here and here.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

Tests are already written for orphan_job_artifact_files.rb but there clearly exists a gap in testing if this was not caught.

Closes #211950 (closed)