Adding broker cleaner script to post-0.16.0 install

[n3wscott]

Fixes #3225

Proposed Changes

• Delete stray Deployments in every namespace owned by a Broker with ChannelBroker class.
• Delete stray RoleBindings and ServiceAccounts from namespaces that had a Broker with ChannelBroker class.
• Delete scaled to zero deployments for mt-broker inside of knative-eventing namespace (we renamed the deployments)

Release Note

Optionally apply <release path>/post-install-to-v0.16.0.yaml to clean up left over Broker resources in all namespaces.

Docs

• Add release notes and install instructions for post-0.16.0 install docs#2624

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: n3wscott

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• OWNERS [n3wscott]

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[mattmoor]

Produced via:
prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github | grep -v docs/cmd/)

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[matzew]

/home/prow/go/src/knative.dev/eventing is out of date. Please run hack/update-codegen.sh

[matzew]

Perhaps it is just me, but I don't understand why this is in cmd/broker-juggler

I'd think it is more reasonable to be in something like cmd/upgrade, like we had before:
https://github.com/knative/eventing/tree/v0.15.0/cmd/upgrade

[n3wscott]

/hold

New updates are being validated with my script.

[n3wscott]

/unhold

Looks good

[vaikas]

/lgtm
/approve

Thanks! neat about dumping all the cleaned resources in yaml log 👍

[vaikas]

/lgtm