[0.18] Fix KafkaBinding conversion

[aliok]

Proposed Changes

• Cherrypick https://github.com/knative/eventing-contrib/pull/1651 into 0.18 branch
• No fuzzing tests like KafkaSource. Created https://github.com/knative-sandbox/eventing-kafka/issues/145 for that

Release Note

- 🐛 Fix bug
Fix KafkaBinding conversion

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aliok

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Needs approval from an approver in each of these files: - ~~[kafka/OWNERS](https://github.com/knative/eventing-contrib/blob/release-0.18/kafka/OWNERS)~~ [aliok] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
kafka/source/pkg/apis/bindings/v1alpha1/kafka_conversion.go	86.7%	85.7%	-1.0

[lionelvillard]

/lgtm