You don’t need to generate the keys on the Ubuntu 18 host (which really needs to be replaced anyway, unless you are paying for extended support). You can copy keys to it that were generated elsewhere.
Can you share the output of the message that you are encountering so that we can have a better context of your issue?
You don’t need to verify your SSH keys unless you are using them to sign commits (git commit -S -m "blah") like you would with a GPG key. Yes, you can configure Gitea to use SSH keys without verifying them.