Fix cloudevent-nodejs sample to support SDK 4.0

[csantanapr]

Closes #3247

The following PR #3136 updated the cloudevent SDK from version 3.x to 4.x and this is a major semantic version, which caused code brakes as the code was not compatible with the new major version of the SDK.
https://github.com/knative/docs/pull/3136/files#diff-8d77ef3aae2dea7cc68b9f32a40ac8f43a211f8f6e7f673d12723ed596eca805R16

Proposed Changes

This PR updates the sample code to be compatible with the new major version of the SDK

[csantanapr]

/assign @vaikas
/assign @knative/api-core-wg-leads

[csantanapr]

I guess PRs can't be assigned to @knative/api-core-wg-leads github teams.

Assigning to Knative Serving since this is a sample under serving in the docs.
Hope the WG leads move forward this PR to resolution.
/assign @mattmoor
/assign @dprotaso

pinging Ville just for awareness
/unassign @vaikas
cc @vaikas

[mattmoor]

Try this:

/cc @knative/api-core-wg-leads

[mattmoor]

Perhaps who can be labeled reviewer depends on who has edit access?

/cc @knative/docs-wg-leads

[mattmoor]

/lgtm

[knative-prow-robot]

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