`v2` is the new tracking tag

[jeffwidman]

We're about to cut a new major version of this action, and we don't anticipate any further releases of the v1 line.

So I simply updated the automation to float the v2 tag.

Technically we could make it so it intelligently looks at the release number and updates the appropriate tag, but that'd be a bit more work and we don't need that complexity in this repo right now given our very infrequent cadence of bumping major versions.

As explained in a code comment:

    # We have a choice - defensiveness vs convenience:
    # 1. Be defensive by filtering if the release doesn't look like a normal
    #    version, or if it's a patch release to an older version... the logic
    #    gets tricky quickly. Easiest way to be 100% sure is stop running this
    #    on `release` and instead require a human to manually run this workflow
    #    after they tag a release.
    # 2. Minimize the upfront hassle by assuming every release is a normal
    #    version release and the latest one. Today both are resoundingly true
    #    as this repo isn't that active/busy, so we don't worry about
    #    multiple release branches, pre-releases, etc.
    #
    # For now I've gone with option 2, as it is much more convenient and if we
    # typo something during a release it's easy to fix by immediately tagging a
    # correct release. And if we don't notice the typo, well, in that case
    # requiring a human to manually run the workflow wouldn't have protected us
    # either, we'd have had to filter by only things that look like versions.
    # Anyway, for now this is good enough, and if it gets to be a problem down
    # the road we increase the robustness of this.