Fix the MAINTAINERS record for the certs/ directory

Fix the MAINTAINERS record for the certs/ directory to have the new
keyrings mailing list and also to be authoritative for the sign-file tool

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>