Description of problem:

While trying to set  priorityClassName for affinity-assistant pods using the field `default-affinity-assistant-pod-template`, the field is not added to the pod yaml.

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

 Edited TektonConfig CR to include below field:

pipeline:
    await-sidecar-readiness: true
    default-affinity-assistant-pod-template: |
      nodeSelector: <-------------
        test: pipelines
      priorityClassName: high-priority <------------
    default-service-account: pipeline
    disable-affinity-assistant: false 

Actual results:

affinity-assistant pod yaml picked up `nodeSelector` but not the `priorityClassName`:

spec:
  nodeSelector: <-------------
    test: pipelines
  restartPolicy: Always
  serviceAccountName: default
  imagePullSecrets:
    - name: default-dockercfg-kxdr2
  priority: 0 <------------------------
  schedulerName: default-scheduler
  enableServiceLinks: true 

Expected results:

priorityClassName should get merged to the affinity-assistant pod yaml.

Reproducibility (Always/Intermittent/Only Once):

Acceptance criteria: 

Definition of Done:

Build Details:

Additional info (Such as Logs, Screenshots, etc):

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