Updating upgrade path

What does this MR do?

Following discussions for ticket (Internal Link), it seems that the unique index that will make sure that there’s no duplicate npm packages in the database was added in 16.3, while the required background migration to mark duplicate npm packages for destruction was added in 16.1.

This should make 16.1 a stopping version.

Related issues

Author's checklist

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Reviewer's checklist

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