Users migrating from --region to --regions will cause having old images keep running

[remorses]

Recently flyctl removed --region and renamed --only-regions to --regions in #3514, this will cause many users to try to migrate --region to --regions (as i did myself).

This migration is very dangerous, the behaviour is completely different and will cause having machines with older image versions running side by side with the new machines.

Imagine what someone that was previously using --region may do when the flag is no longer working:

1. User tries to deploy, --region no longer works
2. Finds the new --regions flag, just replaces it
3. The user passes a new region in this flag thinking the command will simply deploy machines in that region, like it did previously
4. The command instead only runs the deployment ignoring regions not passed to --regions, causing the old machines to keep running with the old image version, possibly breaking production

In my opinion --regions should be changed back to --only-regions. I don't think having old versions of the Docker image keep running in other regions is something many people want, it will cause hard to debug issues where old application code keeps running in a non deterministic way.

Also related to #3537 and #3509