Remove "primary key" language

[trevor-scheer]

We refer to @keys as "primary keys" in various places, but @key doesn't align well with the actual definition of a PK so it's a misnomer that might add confusion. We allow multiple keys and varying keys on the same entity from one subgraph to another, whereas there can only be one PK in a database context. In most cases @key will be used as a PK so it can be helpful to build the mental model but it should be clear that that isn't a constraint that @key is bound to.