Warn about outdated ncu + auto update

[mmarchini]

Having an up to date ncu is essential for folks to land PRs following the latest process, but today if someone tries to use ncu with an older version it will work, which might lead to missing checks before landing something. To avoid that, ncu should check if it's running the latest version and warn the user if it's not (potentially with an option to self-upgrade).

[github-actions]

This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.