Give a better error when user tries to combine `required` and `on target delete deferred restrict` on a link

We currently say ``required links may not use `on target delete deferred restrict` `` but it would be nice to also give the reasoning.

Elvis provided this context:
> Because doing so results in an unsound schema: you delete a target of a link in a transaction, and queries to the end of the transaction would observe an empty value on a required link, which is a violation of the model.

Slack thread for context: https://edgedb.slack.com/archives/CNECHA9EW/p1713959204425729