Use pushing/merger user for issue automatic closing

What does this MR do and why?

Before we would use a commit's author to perform some actions in the ProcessCommitWorker. But, switching those checks to the user pushing the commits as that user can't be spoofed

From #545166 (comment 2564316480) I was under the impression that merging an MR that automatically closes issues would also use the commit's author as the actor for closing issues, but I realize now that we cannot do that as we won't be getting commit information when merge happens, we simply fetch closing issues from the DB that have been configured in the MR description using the closing pattern, or in the commit messages of commits in an MR. So, no change in behavior here 🎉 just what's described above when pushing directly to a default branch.

How to set up and validate locally

You can follow the steps described in #545166 (closed)

After this change, the user closing the issues on commit push will be the user pushing the commits, regardless of the author email set on the commit information (which can be spoofed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #545166 (closed)