[CDF-26473] 🐛Only allow download public canvas (#2281)

# Description

We want to have consistency between Charts and Cavnas in that only
public ones can be downloaded/migrated.

If you have private Canvas - we require you to make them public

## Bump

- [x] Patch
- [ ] Skip

## Changelog

### Fixed

- Running `cdf data download canvas`/`cdf migrate canvas` in interactive
mode only allows selecting public Canvas.
