Don’t create new records on nested attributes

What does this MR do?

The concern is highlighted in https://gitlab.com/gitlab-org/growth/product/-/issues/105#note_402154535. The result is that we've created a couple hundred new gitlab_subscription records, and so we'll need to get this in and then clean up the duplicate records. The problem shouldn't be ongoing until we turn on the new processing fully, so we've limited this to a pretty minor segment.

Are customers impacted in any way due to this? Nah, the impact is mostly limited to the fact that we won't have the history of this most recent update (which was actually a create) for a couple hundred records.

Screenshots

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