Automatically rebase MR when target branch is automatically retargeted

What does this MR do and why?

When we automatically retarget chained merge request's, it can lead to incorrect diffs that take into account the targeted branch, and it self. We can avoid this by rebasing at the same time as retargeting to ensure the merge request has the latest diff.

To Test:

• Create a MR targetting main
• Create another MR targetting the above
• Merge the MR targetting main
• Ensure that the other MR targets main, and was rebased

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #323329 (closed)