fix(controller): add retry-on-conflict to MimirRule reconciler

[Hy3n4]

Summary

• Wrap all r.Update() and r.Status().Update() calls in retry.RetryOnConflict with re-fetch
• Prevents transient "the object has been modified" errors when the SLO controller and MimirRule controller race to modify the same resource
• Follows standard Kubernetes controller pattern for optimistic concurrency handling

Fixes noisy but harmless conflict errors observed in production logs.