Consider publishing version markers

[hasheddan]

This is a companion issue to crossplane/crossplane#2257, which suggests that we should publish version markers to make identifying various "rolling named versions" in a channel more identifiable. We currently do a similar practice of publishing a current build for each channel, but it only allows for, well, the current latest.

From Crossplane issue:

We could consider publishing "version markers", which are essentially just files that use a vanity name for releases. For instance, we could publish a stable-1.txt which contained the version for the latest patch on the latest minor, and a stable-2.txt and stable-3.txt for the latest patches on the two latest patches on the previous two minors. Similarly, we could also have latest-1.1, latest-1.0, etc. markers. These would be auto-updated in the release process, and would make it easier to identify and consume the latest patch for various maintained versions. It would also enhance automated testing as versions can be referred to generically (i.e. test upgrading from stable-1 to stable-2 rather than having to update the explicit versions in the tests).