Make single Ubuntu instructions

We're currently maintaining instructions for each Ubuntu release. This was necessary when our instructions were different for each version of Ubuntu, but now they are all the same. On top of that, having separate drop downs for each OS clutters the list and causes us to have to manually update our instructions each time a new version of Ubuntu is released. For instance, as I'm writing this there are currently no instructions for Ubuntu 18.10.

I think we should have a single Ubuntu dropdown. To do this, I think we should do something similar to what was done for Fedora. See https://github.com/certbot/website/issues/323. If anyone needs any help with that, let me know.

We can also add a note to the top of our Ubuntu instructions if we want saying that only non-EOL'd versions of Ubuntu are supported.

@schoen, I think this would be another useful thing for you to work on if you're interested.