For JENKINS-40370, we'll be doing new syntax for when. Given certain similarities to agent, it makes sense to have a common look and feel to the two sections' syntax, and we already had an interest in making the agent syntax more flexible going forward, so let's move from

agent label: 'foo'

to

agent {
  label 'foo'
}

We'll still support agent none and agent any, since they're useful shortcuts that we've already got special logic for.