Use ssh_signature.committer_email and gpg_signature.committer_email columns to determine signed UI commit committer has been mailmapped

What

Use ssh_signature.committer_email and gpg_signature.committer_email columns to properly determine if a committer email from a signed UI commit has been mailmapped.

Why

An MR exists which attempts to do this for ssh_signature.author_email and gpg_signature.author_email. After a discussion in https://gitlab.com/gitlab-org/gitlab/-/issues/425042#note_2506140631 it was determined that we should be using an unmailmapped committer email for this check not author email. We should update the check in that MR to use committer email.