feat: milestone verification workflow

[Amnah199]

Every Haystack release is tied to a GitHub milestone that tracks issues and pull requests that must be closed before releasing the new version. Currently, these milestones are manually tracked.

Proposal Add a GitHub Actions workflow that automatically checks whether the current release milestone has any open issues or PRs. If any remain open, the workflow should fail and block the release process until they are resolved.

Requirements

1. Determine the current release version: Read the version from haystack/version.py (or equivalent).
2. Compute the next release milestone name: Format: v.. (e.g. v1.2.3).
3. Query GitHub’s API for open issues/PRs: Fail if any open items exist.

Criteria

• [ ] A new workflow file (.github/workflows/check-milestone-closure.yml).
• [ ] The workflow runs on demand (manual dispatch) and as a required check before tagging a release.
• [ ] If the milestone has open issues or PRs, the workflow exits with a non‑zero status and prints a list of open items.
• [ ] If all items are closed, the workflow passes successfully.
• [ ] Documentation is updated (in notion) to reference the new automated check.