Handle unsupported images better at launch

[ricab]

What are you trying to do?
multipass launch 16.04 behaves pretty much the same as multipass launch lts, except that the former fails to complete, because the image is no longer supported.

We decided a while ago to scope such images out of what Multipass is supposed to support. See #3443 and #1068 for reference. However, the UX is not great today. People keep trying this, seeing it fail in an ungraceful way, which must be disappointing.

What's your proposed solution?
I propose Multipass does one of 3 things:

1. Stop offering unsupported images entirely (people could still launch them from files or URLs directly).
2. Allow launching unsupported images only if the user passes a new --allow-unsupported flag. We should then probably require this flag for any file-/URL-based launches.
3. Warn the user that the image is unsupported. Warnings are not shown by default though, so we'd need to think about how to achieve this.

Since we want to allow file/URL based launches (allowing images that are not supported ubuntu releases) I think my preference would be option 2.

Additional Context

$ multipass find xenial
No images or blueprints found.
$ multipass find --show-unsupported xenial
Image                       Aliases           Version          Description
xenial                                        20211001         Ubuntu 16.04 LTS
daily:xenial                                  20211001         Ubuntu 16.04 LTS