add workflow to close stale issues

[lewisjkl]

Raising this PR to be able to discuss the possibility of adding a workflow like this, I noticed we have some old PRs and I think in general it is nice to focus on PRs/Issues that have active interest/contributions.

Let me know your thoughts.

PR Checklist (not all items are relevant to all PRs)

• [ ] Added unit-tests (for runtime code)
• [ ] Added bootstrapped code + smoke tests (when the rendering logic is modified)
• [ ] Added build-plugins integration tests (when reflection loading is required at codegen-time)
• [ ] Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• [ ] Updated dynamic module to match generated-code behaviour
• [ ] Added documentation
• [ ] Updated changelog