[release-1.14] Report error for unsupported PKCS #1 format for the private key

[knative-prow-robot]

This is an automated cherry-pick of #4114

Report error for unsupported `PKCS #1` format for the private key.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[knative-prow]

@knative-prow-robot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
channel-reconciler-tests-sasl-ssl_eventing-kafka-broker_release-1.14	1046717	link	true	/test channel-reconciler-tests-sasl-ssl
integration-tests_eventing-kafka-broker_release-1.14	1046717	link	true	/test integration-tests

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