Fix handling for getting latest stable tag (#62858)
This simplifies our `getLastStable` handling to use our GitHub latest
release as the source of truth instead of trying to parse this from the
git history as that can be less reliable depending on the checkout depth
and the order of releases.
x-ref:
https://github.com/vercel/next.js/actions/runs/8149447058/job/22274305183
Closes NEXT-2704
