Request for a `release/latest` branch or tag

[bonachea]

This issue requests creation and maintenance of a git ref (branch or tag, don't really care which) that always points to the latest stable public release of Chapel. The existence of such a ref helps git users to checkout the latest stable release of Chapel, without needing to know the exact version number.

The main reason I want this is to enable adding a git branch alias to the Chapel Spack formula that explicitly requests fetching the current public release from git (e.g., spack install chapel@stable). We could implement such an alias by updating the alias in the Spack formula after each public release, but that's not ideal because the update might not reach some Spack users for months after the Chapel release. Also, maintaining the ref in git would be more helpful to more people (i.e., those not using Spack).

I have no strong opinion on the naming, but release/latest seems to match the current naming convention in the Chapel repo. Other common alternatives could include stable or simply latest.

This ref would need to be updated during the Chapel release process, either by merging the release to a long-lived branch, or by moving the tag. This need not happen on release day, in fact there might be some benefit in performing this update after code freeze but before the release is officially announced.

CC: @bradcray @arezaii