Combine Ubuntu 19.10 and Ubuntu 20.04 robustly

With https://github.com/certbot/website/pull/583, we can combine these instructions again. I think doing this is valuable to reduce the number of dropdown options.

To do this, we can:

1. Ask tech ops to set up a HTTP redirect from Ubuntu 20.04 to Ubuntu 19.10.
2. Delete the Ubuntu 20.04 dropdown option and update the text for `ubuntuother` to say Ubuntu 19.10+.

I started to just do this, but I worry the presence of external redirects like this could create bugs in the future. We already have one redirect like this from `fedora24*` to `fedora`. The redirect for ancient fedora versions seems unlikely to create bugs to me, but I worry creating this Ubuntu redirect could cause problems. Ubuntu 20.04 will be around for a long time and if we ever need to split out our Ubuntu 20.04 instructions again, users will be redirected to our Ubuntu other instructions and this will not be apparent when testing the site. Maybe we should try to get `nginx.conf` in the root of this repo in sync with tech ops version? One way to do this might be to include it in the files pushed to `website-builds` which would also help https://github.com/certbot/website/issues/579.