test-infra: plank reports cannot apply changes reported at error level

It seems like this is an expected state of affairs? Since crier etc can also update the object.

Can we report this at a info level unless its a consistent problem?

/area prow /area prow/plank /kind oncall-hotlist

/cc @stevekuznetsov @cjwagner

{
 insertId: "1fmajjkg10u0n5f"  
 jsonPayload: {
  component: "plank"   
  error: "errors syncing: [Operation cannot be fulfilled on prowjobs.prow.k8s.io "7709fdf4-eb9a-11e9-9232-aacf8944b3c6": the object has been modified; please apply your changes to the latest version and try again], errors reporting: []"   
  file: "prow/cmd/plank/main.go:144"   
  func: "main.main.func2"   
  level: "error"   
  msg: "Error syncing."   
 }
 labels: {…}  
 logName: "projects/k8s-prow/logs/plank"  
 receiveTimestamp: "2019-10-10T20:14:24.103165269Z"  
 resource: {
  labels: {
   cluster_name: "prow"    
   container_name: "plank"    
   instance_id: "6617373689885452859"    
   namespace_id: "default"    
   pod_id: "plank-6655fd65bf-t88s5"    
   project_id: "k8s-prow"    
   zone: "us-central1-f"    
  }
  type: "container"   
 }
 severity: "ERROR"  
 timestamp: "2019-10-10T20:14:18Z"  
}