Rename columns author_email on ssh_signature and gpg_signature tables to committer_email

What

Rename columns author_email on ssh_signature and gpg_signature tables to committer_email.

Why

Previously we add a author_email column to the gpg_signatures and ssh_signatures table respectfully to store the original author of a UI commit signature pre mailmap. Following a discussion in https://gitlab.com/gitlab-org/gitlab/-/issues/425042#note_2506140631 it was determined that we should be checking committer email not author email. We should update the names of these columns to store the committer email instead.