Since #171 was merged, this action no longer touches `known_hosts`; this
line should have been removed from the README at that time.

In this PR, I update the `actions/checkout` versions. When I recently
started using this action, I was confused about whether it works with
the newest checkout action. I tested it and everything is fine. For
future users, we should display examples with the newest versions.

Fix for deprecated node16

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: webfactory/ssh-agent@v0.8.0. For more information
see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/