a8n: Deprecate CampaignPlan.changesets in favor of CampaignPlan.changesetPlans

Created by: mrnugget

This is a follow-up to #7965 and stacked on top of it. See motivation/reasoning in #7965.

I propose the change here because the returned type is a ChangesetPlansConnection and not Changesets. The same field is called changesetPlans on Campaign.

Update: Decided to only deprecate the field, since it's heavily used the src CLI which is now being used by customers.