insights: require the entire dashboard in the update mutation
Created by: CristinaBirkel
Context: https://github.com/sourcegraph/sourcegraph/pull/26591#discussion_r735924741
Our mutation for updating dashboards doesn't currently require all fields, and will only update the fields that are populated in the request:
This pattern will be cumbersome and complex for us to repeat for insights and other potential update mutations, so in order to keep behavior consistent across the API we should remove this "nice to have" functionality from the dashboard update. Our current front end should not be using this anyway, so it should be safe to rip out, but do double check.