FR: create git tags and github releases only when a supported release is done

[a-crate]

Currently there are lot of github releases and git tags, but these don't corresponding to a supported and released version because no binary release is done by the team at Google.

This is confusing for users, and difficult to mange for distributions packaging the guest-agent as this package has to be versioned manually.

This FR is a request to only create tags and releases when a version is actually released and supported (or least, a machine parse-able list of supported versions somewhere in this repository).

[a-crate]

A sub-request of this is that releases done from topic-stable be distinguishable in some way from releases done from main, so that it is possible to follow either release track. Currently these tags are all mixed together, but that's irrelevant since those tags don't represent a supported release anyway.