gitlab-psql system user is hardcoded in pg_upgrade.rb

The system user gitlab-psql is hardcoded in the migration of postgresql. Automigration doesn't work for installations where a different user is configured in the postgresql['username'] setting of gitlab.rb.

https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-ctl-commands/lib/gitlab_ctl/pg_upgrade.rb#L31

def run_pg_command(command)
  GitlabCtl::Util.get_command_output("su - gitlab-psql -c \"#{command}\"")
end

As a workaround, we had to disable auto migrations and edit the relevant portion of pg_upgrade.rb to force usage of our configured user before running the automigration manually