Add validation to ObservabilityPolicy to require TargetRefs

[kate-osborn]

The ObservabilityPolicy CRD should require a minimum of 1 TargetRef.

See this convo: #2515 (comment)

• Will need to bump version of CRD
• Confirm that bumping the version of the CRD does not affect valid ObservabilityPolicys
• How do we handle v1alpha1? Deprecation for 3 weeks? Look at Kubernetes doc on CRD versioning and update our own guide.
• Just follow the doc on how to do versioning changes

[kate-osborn]

During refinement let's discuss whether this mandates a version bump