fly machine update --volume

# fly machine update --volume

fly machine update --volume no longer attaches volumes. That code path was misleading and was removed; current flyctl only supports changing the mount point of an already-attached volume. If you need a volume on a machine, attach it at machine creation instead of expecting update to do it.

## Context from the issue
gh:superfly/flyctl#1605: Issue superfly/flyctl#1605 (closed, 9 comments): **Describe the bug** Running this command will show an expected diff and the output will show the machine successfully updated. But the volume doesn't get attached to the machine. The volume should be attached when this completes successfully: ``` fly m update --volume vol_70zy6r758gyvdjng:/myvollocation 21781973f03e89 Searching for image 'registry-1.docker.io/library/nginx:latest' remotely...