New fields should only need a top level introduced_at

[tonysimpson]

acceptable lint is asking for an introduced_at on all nodes under a new property. Only the top level should need one when its new.
Errors on:
"common-ids": {
"type": "array",
"items": {"type": "string"},
"introduced_at": 20,
},
Wants:
"common-ids": {
"type": "array",
"items": {"type": "string", "introduced_at": 20},
"introduced_at": 20,
},