Improve MR Retarget Chain to prefer usage of `MR::DeleteSourceBranchWorker`

What does this MR do?

Related to:

• #320895 (closed)
• #321559

This fixes the outstanding problems handling user choice of whether to delete or not source branch:

1. If a MR has a remove source branch saved, even unchecking that on right hand of Merge button will still retarget the MR
2. If a MR does not have a remove source branch saved, even checking that on right hand of Merge will not retarget the MR

This does not solve:

1. Clicking Remove Source Branch after MR is merged calls DELETE /branches and thus does not trigger retarget

Does this MR meet the acceptance criteria?

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