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

[knative-prow-robot]

This is an automated 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`.

[knative-prow-robot]

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[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    #5904   +/-   ##
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  Coverage                ?   82.68%           
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  Files                   ?      203           
  Lines                   ?     6346           
  Branches                ?        0           
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  Hits                    ?     5247           
  Misses                  ?      759           
  Partials                ?      340           

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[knative-prow-robot]

@knative-prow-robot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-knative-eventing-build-tests	e00b6bb	link	true	/test pull-knative-eventing-build-tests

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[pierDipi]

broken due to Go 1.17: see #5906