consider deploying ARC on lists-01

in #40471 (closed), we focused on upgrading the mailman server from legacy, which we did. in doing so, we kept some of the current warts, which is that we don't use ARC to sign incoming headers.

i briefly considered just flipping the switch on this as Mailman itself has direct support for ARC signatures (see the [arc] configuration section), but i'm a little hesitant in doing so because I feel it might be better to process this out of band, similarly to other servers, for example the MX (#40987)