Choose simplestreams releases to display automatically

[ricab]

What are you trying to do?
On Windows and macOS, Multipass should decide what releases it supports dynamically and automatically, rather than requiring us changing that in the code and having a new release.

What's your proposed solution?
There should be no need to manually update private-side filters with hardcoded release names. Instead, Multipass should dynamically select and show any LTS + the latest release on these platforms (these may be the same release). Releases can be ordered based on their version number.