Is your feature request related to a problem? Please describe.
Ajour settings allows only to get the latest version from the selected channel. Some addon developers do not use alpha or beta channels which mean ajour would never show an update is available, even though it is on a higher channel.

Describe the solution you'd like
There should be a fallback mechanic to check for updates. e.g. if alpha is selected and there are no alpha version, check for beta version instead, if no beta versions check for stable ones.

Additional context
This simple mechanic would not work for irregularly updated addons, e.g. higher version being pushed directly to stable and not to beta/alpha but this may be a bit more complex in regards to detection.