Rename or delete "staging" and "production" stages

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [ ] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

It would be nice to be able to rename the default stages, or alternatively to remove them. My company would like to use dev, stg and prod, as stage names, and having "staging" and "production" alongside "prod" and "stg" will only cause confusion.