Mark Virtualbox backend as deprecated

With the new Hyper-V driver, the Virtualbox driver will be deprecated.

If the old Virtualbox driver is active, Multipass should

- in the GUI: Show a note/warning : "Your current driver is deprecated. _Learn more_." such thath when the user is clicking on the note/warning and on "Learn more" a documentation page is opened that shows content similar to https://canonical.com/multipass/docs/latest/how-to-guides/customise-multipass/migrate-from-hyperkit-to-qemu-on-macos/

- When using _any_ CLI command, display a deprecation message above any output:

```
[warning] [daemon] The virtualbox driver is deprecated and will be removed in an upcoming release.
There is no automatic migration path planned.
We encourage to switch to the new hyperv-api driver (multipass set local.driver=hyperv-api) as soon as possible.
```