Pin to latest pubsub protocol to get bug fix

[liu-cong]

Fixes #1178

Proposed Changes

• Pull in bug fix in sdk-go Fix pubsub message nil pointer due to missing format cloudevents/sdk-go#528

Release Note

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liu-cong

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• OWNERS [liu-cong]

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[grac3gao-zz]

/lgtm

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/broker/handler/handler.go	85.7%	86.4%	0.6
pkg/broker/handler/processors/deliver/processor.go	83.3%	84.1%	0.8
pkg/broker/ingress/handler.go	90.9%	91.2%	0.3

[nachocano]

/lgtm

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-google-knative-gcp-integration-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-google-knative-gcp-integration-tests:

github.com/google/knative-gcp/test/e2e.TestChannelTracing
github.com/google/knative-gcp/test/e2e.TestChannelTracing/includes_incoming_trace_id

[liu-cong]

/retest