Do not re-create subscription on sequence update

[joke]

Describe the bug
If a sequence is updated with a new retry configuration the subscription is re-created.

In case of a KafkaChannel a new consumer group is created. Thus messages might be dropped.

Expected behavior
The subscription is updated in-place rather than re-created.

To Reproduce
Knative Kafka Broker needs to be installed: https://github.com/knative-extensions/eventing-kafka-broker

Create sequence:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: first
spec:
  template:
    spec:
      containers:
        - image: gcr.io/knative-releases/knative.dev/eventing/cmd/appender
          env:
            - name: MESSAGE
              value: " - Handled by 0"

---
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: second
spec:
  template:
    spec:
      containers:
        - image: gcr.io/knative-releases/knative.dev/eventing/cmd/appender
          env:
            - name: MESSAGE
              value: " - Handled by 1"
---
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: third
spec:
  template:
    spec:
      containers:
        - image: gcr.io/knative-releases/knative.dev/eventing/cmd/appender
          env:
            - name: MESSAGE
              value: " - Handled by 2"
---
apiVersion: flows.knative.dev/v1
kind: Sequence
metadata:
  name: sequence
spec:
  channelTemplate:
    apiVersion: messaging.knative.dev/v1beta1
    kind: KafkaChannel
    spec:
      numPartitions: 1
      replicationFactor: 3
  steps:
    - ref:
        apiVersion: serving.knative.dev/v1
        kind: Service
        name: first
      delivery:
        backoffDelay: PT1S
        backoffPolicy: linear
        retry: 5
    - ref:
        apiVersion: serving.knative.dev/v1
        kind: Service
        name: second
      delivery:
        backoffDelay: PT1S
        backoffPolicy: linear
        retry: 5
    - ref:
        apiVersion: serving.knative.dev/v1
        kind: Service
        name: third
      delivery:
        backoffDelay: PT1S
        backoffPolicy: linear
        retry: 5

The subscriptions should have been created.

$ kubectl get subscription
NAME                     AGE   READY   REASON
sequence-kn-sequence-0   14s   True    
sequence-kn-sequence-1   14s   True    
sequence-kn-sequence-2   14s   True    

Modify the sequence and update the delivery spec on step 2 -> retry: 7 or something

$ kubectl get  subscription     
NAME                     AGE     READY   REASON
sequence-kn-sequence-0   4m9s    True    
sequence-kn-sequence-1   5s         True    
sequence-kn-sequence-2   4m9s    True

The second subscription is re-created.

Knative release version

Knative 1.13 and below. Most likely 1.14 is affected as well.

[Cali0707]

/triage accepted
/assign