"verification-mode" field should be removed from pipelines spec

Originally verification-mode from pipelines spec was removed in https://github.com/tektoncd/operator/pull/1481, but upgrade blocked with the following error,

{"level":"info","logger":"tekton-operator-webhook","caller":"webhook/admission.go:151","msg":"remote admission controller audit annotations=map[string]string(nil)","commit":"8e8c163","knative.dev/kind":"operator.tekton.dev/v1alpha1, Kind=TektonConfig","knative.dev/namespace":"","knative.dev/name":"config","knative.dev/operation":"UPDATE","knative.dev/resource":"operator.tekton.dev/v1alpha1, Resource=tektonconfigs","knative.dev/subresource":"","knative.dev/userinfo":"system:serviceaccount:openshift-operators:openshift-pipelines-operator","admissionreview/uid":"958be8bd-9d63-4b95-aa7d-9285684ff86c","admissionreview/allowed":false,"admissionreview/result":"&Status{ListMeta:ListMeta{SelfLink:,ResourceVersion:,Continue:,RemainingItemCount:nil,},Status:Failure,Message:mutation failed: cannot decode incoming old object: json: unknown field \"verification-mode\",Reason:BadRequest,Details:nil,Code:400,}"}

hence verification-mode field added back to pipelines spec with the PR => https://github.com/tektoncd/operator/pull/1498

verification-mode should be removed, keep this ticket open until verification-mode removed.

Start by reviewing the pipelines spec and the changes in PRs 1481 and 1498, then trace the webhook admission error for upgrades involving an old object. Done means verification-mode is removed without causing the reported decode failure during an operator upgrade.