[Designer] The payload editor warns of missing version property in nested cards

Problem Description

Using the activity update sample from https://adaptivecards.io/samples/ActivityUpdate.html, the designer shows an error in the JSON:

However, the spec states that version is optional for cards nested within Action.ShowCard. I suppose this is a side-effect of using the JSON schema to validate the code in the editor. The fact that an official sample (the first one to be listed at that) shows up as invalid in the official editor seems rather unfortunate.

Card JSON

{
    "type": "AdaptiveCard",
    "actions": [
        {
            "type": "Action.ShowCard",
            "title": "Set due date",
            "card": {
                "type": "AdaptiveCard",
                "actions": [
                    {
                        "type": "Action.Submit",
                        "title": "OK"
                    }
                ]
            }
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.6"
}

Start with the ActivityUpdate sample linked in the issue and reproduce the warning in the Designer using the provided card JSON. Trace the editor's JSON-schema validation for the nested Action.ShowCard card and compare it with the adaptive-card.json schema and explorer specification. Done means the official sample validates without incorrectly requiring version on the nested card.