[release-0.26] Default dead letter sink namespace to the object namespace

[pierDipi]

This is a cherry-pick of #5748

The field `spec.delivery.deadLetterSink.ref.namespace` for Broker, Trigger, Channel and Subscription if not specified is defaulted to `metadata.namespace`.

The automated cherry-pick is broken due to Go 1.17

Reproducer:

---
apiVersion: v1
kind: Namespace
metadata:
  labels:
    bindings.knative.dev/include: "true"
  name: broker-v1-dlq
---
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: event-display
  namespace: broker-v1-dlq
spec:
  template:
    spec:
      containers:
      - image: quay.io/openshift-knative/knative-eventing-sources-event-display:v0.13.2
---
apiVersion: messaging.knative.dev/v1
kind: Channel
metadata:
  name: dlq-channel
  namespace: broker-v1-dlq
---
apiVersion: eventing.knative.dev/v1
kind: Broker
metadata:
  name: broker
  namespace: broker-v1-dlq
spec:
  delivery:
    backoffDelay: PT1S
    backoffPolicy: linear
    deadLetterSink:
      ref:
        apiVersion: messaging.knative.dev/v1
        kind: Channel
        name: dlq-channel
    retry: 5
---
apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
  name: subscription
  namespace: broker-v1-dlq
spec:
  broker: broker
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: event-display
---
apiVersion: sources.knative.dev/v1
kind: SinkBinding
metadata:
  name: bind-heartbeat
  namespace: broker-v1-dlq
spec:
  subject:
    apiVersion: apps/v1
    kind: Deployment
    selector:
      matchLabels:
        app: heartbeat
  sink:
    ref:
      apiVersion: eventing.knative.dev/v1
      kind: Broker
      name: broker
  ceOverrides:
    extensions:
      sink: bound
---
apiVersion: eventing.knative.dev/v1beta1
kind: EventType
metadata:
  name: dev.knative.eventing.samples.heartbeat
  namespace: broker-v1-dlq
spec:
  type: dev.knative.eventing.samples.heartbeat
  source: https://knative.dev/eventing-contrib/cmd/heartbeats/#broker-v1/heartbeat
  broker: broker
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hearbeat
  namespace: broker-v1-dlq
  labels:
   app: heartbeat
   bindings.knative.dev/include: "true"
spec:
  selector:
    matchLabels:
      app: heartbeat
  template:
    metadata:
      labels:
        app: heartbeat
    spec:
      containers:
      - name: single-heartbeat
        image: quay.io/openshift-knative/knative-eventing-sources-heartbeats:v0.13.2
        args:
        - --period=1
        env:
        - name: POD_NAME
          value: heartbeat
        - name: POD_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace

Output:

    - lastTransitionTime: "2021-11-15T08:19:19Z"
      message: channels.messaging.knative.dev "dlq-channel" not found
      reason: Unable to get the dead letter sink's URI
      status: "False"
      type: DeadLetterSinkResolved

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/broker/trigger/trigger.go	88.5%	88.0%	-0.5

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.26@27dbb99). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             release-0.26    #5907   +/-   ##
===============================================
  Coverage                ?   82.68%           
===============================================
  Files                   ?      203           
  Lines                   ?     6346           
  Branches                ?        0           
===============================================
  Hits                    ?     5247           
  Misses                  ?      759           
  Partials                ?      340           

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[antoineco]

/lgtm

[knative-prow-robot]

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