Add a suggestion/warning when running "registry list" for the first time since behaviour change

[philrenaud]

If you run nomad-pack registry list after upgrading to the latest upcoming release, you'll be met with a list of packs that you, since #354 , would now expect to be seen with nomad-pack list, and with columns missing:

The user can fix this by running nomad-pack registry delete default (or whatever non-default registry they happened to have), but there's no indication that this is required when they first run the new registry list. This could be confusing for existing pack users.

Could we detect if registry list lists packs, not registries, as indicated by the above screenshot, and suggest to the user that they run a registry delete ______ along a message that the command has changed? If this message gets shown conditionally when getting the above column-missing output, it seems like it would be unobtrusive but helpful.