feat: automate next milestone creation

[Amnah199]

Background Currently we create new GitHub milestones for upcoming Haystack releases manually.

Proposal Add a GitHub Actions workflow that, when triggered, automatically reads the current release version, computes the next semantic version, and creates an empty milestone in the repo.

Criteria

• [ ] A new workflow file at .github/workflows/create-next-milestone.yml.
• [ ] Workflow is triggered via workflow_dispatch (manual trigger).
• [ ] Reads the current Haystack version (e.g. 2.7.0) and calculates the next version by bumping the minor field and resetting patch (e.g. → 2.8.0).
• [ ] Creates a GitHub milestone titled v2.8.0 if it does not already exist.
• [ ] If the milestone already exists, the workflow exits cleanly with a notice (no failure).
• [ ] Prints the new milestone’s URL or name on success.
• [ ] Document the new workflow in notion.

[julian-risch]

I am putting P3 as priority because we already have milestones created for 2.13, 2.14, which means we are good for the next 8 weeks. It's rare that we need the 2.x+1 milestone only after kicking off the release process for 2.x and I would expect this automation exit most of of the time without doing anything because the new milestone already exists. Automation is nice to have but the creation of the next milestone is neither time‑consuming nor error‑prone I'd say.