Resolve "Background Migrations Visibility Improvement for GitLab engineers - Fix progress value"

What does this MR do and why?

The system estimates the number of rows in a table to calculate the batched background migration progress. Since this is not an exact value, sometimes we have the migration finished, but the progress is not 100%. This can confuse the developers.

Example:

To mitigate this, we can return "100" when the migration is finished.

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MR acceptance checklist

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Related to #346357 (closed)