Rename STUNner charts to reflect functions better

Historical overview: we started with a single `stunner` chart, then came the `stunner-gateway-operator`.  Currently, both exist, and this can be misleading for new users. 

Proposed changes:
- [ ] rename the current `stunner` chart as `stunner-unmanaged`
- [ ] make `stunner` an alias for the current `stunner-gateway-operator`
