Tech Debt removal: Update upload_to_harbor.py to use Pathlib and shutil.copy

Summary

During the review of !935 (merged), it was noticed that we are not using pathlib to create file paths (rather than string interpolation), and using shutil.copy, rather than read/write buffers to move file contents from one location to another.

Details

The main area of focus starts here, but the entire file should be reviewed to ensure consistency.