git_ui: Capitalize co-author prefix (#23365)

This PR capitalizes the co-author prefix, as this is the way GitHub
writes it in their
[docs](https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors#creating-co-authored-commits-on-the-command-line).

Release Notes:

- N/A
8bd49b0966Marshall Bowers committed on 1/20/2025, 4:00:47 PM· committed by GitHubparentd0db059
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