Follow-up from "Automatically retarget merge requests [RUN ALL RSPEC] [RUN AS-IF-FOSS]"

The following discussions from !53710 (merged) should be addressed:

• @ayufan started a discussion: (+7 comments)

  Fix Delete Source Branch when clicked manually by user

• @pedroms started a discussion: (+1 comment)

  @ayufan 📝 Text: thank you for adding this separate message for automatic retargeting 🙇 I have a few notes for your appreciation:

  1. Per !53710 (comment 507810614), it would be ideal to communicate that old_branch was deleted in another merge request so that the new_branch selection makes just a little bit more sense. As a developer that just stumbled upon this new kind of system note, would the following make more sense to you?
     1. Name (@username) automatically changed #{branch_type} branch to #{new_branch}because#{old_branch} was deleted in another merge request
  2. Consider moving automatically so the sentence reads a bit better: “automatically changed automatically target branch…”