Issue.Grounds changed from a string to a list in the json envelope on an unreleased branch

Issue.Grounds changed from a string to a list in the json envelope on an unreleased branch

Reported by the round-5 builder. Issue.Grounds moved from string to []string in the JSON envelope when grounds became append-only.

No in-tree consumer, and the field shipped on this same unreleased branch, so no released surface moves and this is not a break. Recorded only so that the shape change is findable if anything external bound to it early, and so the next release note does not have to rediscover it.

Also noted by the same review and folded here rather than given an id: capture importing intent for CreateDraft/Buckets remains, which is pre-existing and unrelated to this round.

Grounds

• pursued: the shape change is findable and broke no released consumer; shown wrong if an external consumer is found bound to the string-shaped Grounds field