BitBucket Server Importer - Preserve PR (MR) reviewers

What does this MR do and why?

This MR is a 16.2 back-port from BitBucket Server Importer - Preserve PR (MR) re... (!126729 - merged)

Screenshots or screen recordings

Refer to the main MR!

How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416611 (closed)