Frontend: Rename commit target branch to avoid confusion

What does this MR do and why?

For #351459 (closed)

The branch terminology for the commit form in the editor was confusing. Since we typically refer to branches that we are committing on as source branches, I've updated the variables to reflect this.

Because we can commit to a source branch or master (typically referred to as our target branch), we've removed the defining terminology and have updated our form to simply say "Branch".

How to set up and validate locally

1. Visit the Pipeline Editor (CI/CD > Editor)
2. When viewing the commit form at the bottom, the field under "Commit message" should be named "Branch" instead of "Target Branch". The relevant variables have also been renamed.

Screenshots or screen recordings

Before	After

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