Update the Go version used for Ubuntu in the README

[paperclipstudio]

From here --> #115 (review)
Just putting this here as a reminder, for me or anyone else who wants to support.

[TeknoVenus]

In fact, it looks like umoci is available in the Ubuntu package repositories from 20.04 onwards, so can just update instructions to pull it from there: https://packages.ubuntu.com/jammy/umoci.

The main reason for explicitly using Go was because Ubuntu 18 didn't ship with umoci so we had to build from source.