Use milestone and ignore merge status

[jni]

When it's close to release date we make a lot of add/remove from milestone decisions quickly. With the current state of the release script, we have to use --with-pr many times to signal intent to merge. And, as new PRs are added or removed from the milestone, the invocation of the script needs to change each time.

Instead, the milestone itself should be the signal of intent. That way, we can just invoke the script repeatedly as milestones are edited. The milestone itself becomes the single source of truth, and using the script is simpler.