Set CF_INSTANCE_INDEX based on statefulset.kubernetes.io/pod-index label

[tcdowney]

⚠️ BLOCKED UNTIL VERSION 1.27 FALLS OUT OF SUPPORT ⚠️

Background

Kubernetes version 1.28+ will set the statefulset.kubernetes.io/pod-index label on Pods within a StatefulSet. We should be able to set the CF_INSTANCE_INDEX env var on Pods with the value of that label using the K8s Downward API instead of our custom webhook.

Action to take

• Update the statefulset-runner (and inform users who may be using custom runners) to set CF_INSTANCE_INDEX based on the new pod index label
• Remove the webhook that does this job today

Impact

• This will simplify the code and allow us to remove an awkward Pod mutating webhook.
• Question: Will this change cause all StatefulSets to roll at the same time or will it only take effect when a user explicitly restarts an app? Or will it just occur when the statefulset-runner controller resyncs a particular AppWorkload?

Dev Notes

• We can't get rid of the webhook until Kubernetes version 1.27 falls out of support.
• Add StatefulSet pod index as pod label kubernetes/kubernetes#119232