Update stale.yml

[yuxiang-zhang]

Description

Observing quite a few of issues getting closed without getting solved, and the maintainers and rest of the community lost track of them over time: https://github.com/weaveworks/eksctl/issues?q=is%3Aissue+label%3Astale+is%3Aclosed+

This PR exempts a couple of labels to help mark tickets that shouldn't become stale:

• priority/backlog
• needs-investigation

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[a-hilaly]

👍