update config elements for eventing channel and pingsource

[odacremolbap]

"Fixes #issue-number" or "Add description of the problem this PR solves"

Proposed Changes

• Update ConfigMap element names for eventing's channel and pingsource. They are being renamed to keep consistent with other configuration elements.

Previous configuration element names are still processed the same but we want the new ones to be the only reference to users.

MERGEABLE since related PR at knative/eventing#5875 got merged.

[knative-prow-robot]

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[odacremolbap]

/assign @csantanapr

[csantanapr]

/approve
/lgtm

For others to be aware, the change in eventing just got merged, and this doc update should not be cherry-picked.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csantanapr, odacremolbap

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