chore(chart-deps): update knative-operator to version v1.21.0

[svcAPLBot]

This PR updates the dependency knative-operator to version v1.21.0.

[svcAPLBot]

Comparison of Helm chart templating output:

@@ spec.template.spec.containers.operator-webhook.image @@
# apps/v1/Deployment/knative-operator/operator-webhook
! ± value change
- gcr.io/knative-releases/knative.dev/operator/cmd/webhook:v1.18.1
+ gcr.io/knative-releases/knative.dev/operator/cmd/webhook:v1.21.0

@@ spec.template.spec.containers.knative-operator.image @@
# apps/v1/Deployment/knative-operator/knative-operator
! ± value change
- gcr.io/knative-releases/knative.dev/operator/cmd/operator:v1.18.1
+ gcr.io/knative-releases/knative.dev/operator/cmd/operator:v1.21.0


@@ data.core.yaml @@
! ± value change in multiline text (one insert, one deletion)
  adminApps:
  - deps:
    - prometheus
    ingress:
  
  [432 lines unchanged)]
  
    knative:
      about: Knative Serving builds on Kubernetes to support deploying and serving of
        applications and functions as serverless containers. Serving is easy to get
        started with and scales to support advanced scenarios.
-     appVersion: 1.18.1
+     appVersion: 1.21.0
      chartName: knative-operator
      integration: Knative serving can be activated to deliver Container-as-a-Service
        (CaaS) functionality with a scale-to-zero option. It can be compared to Functions-as-a-service
        (FaaS) but is container oriented, and takes only one manifest to configure an
  
  [316 lines unchanged)]
  
      svc: tekton-dashboard
      type: public
    name: tekton
    ownHost: true