Update migration tools

[stonezdj]

The previous migration tool overwrites existing OCI artifacts with the same name and same tag

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[zyyw]

how about change line 28 with open("/chart_storage/migration_errors.txt", "a") as f: in file tools/migrate_chart/migrate_chart.py to NOT append, just write into a file?

Also how about write a timestamp at beginning or end of the output file?

[wy65701436]

lgtm

[Vad1mo]

I wonder hat does this code has to offer vs. https://github.com/goharbor/chartmuseum-migration-tools and why it should be part of the core project.