There's some information about upgrading certbot in general in this thread:
But I don't know if there's much there that will help with an Ubuntu that old. If that OS is new enough to support snap, you can install Certbot through snap, but I don't know if that'll work for you.
The general advice used in these cases (other than to upgrade the server, which you say you're working on) is to switch to a different ACME client, such as one of the shell-script or go based ones, since they have the fewest dependencies and can generally just be copied onto the server and run:
Another option, if you have access to DNS for the zone, is that you could also run certbot on some other newer system in manual dns-challenge mode, get the certificate & private key there, and then securely copy them over the existing files on your old server. Being a manual process you'd need to repeat it every couple months, but if you're decommissioning the server is a couple months it might get you going for now. I don't know as that'd really be easier than switching to a new client on that existing system, though.