[DEPRECATION] Deprecate setting relationships via PromiseProxies

[runspired]

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Users would instead resolve the promise and set value as the actual instance of a record.

A lesser deprecation would only deprecate setting relationships via unresolved promise proxies.

[runspired]

I think we may have already completed a deprecation cycle here, I'll have to review and get back.

Because typescript makes it impossible to have an async getter with a sync setter cleaning up the relationship story is high priority. Likely this issue should be replaced with one proposing a new story for relationships.