Use ghc-prim < 0.7, not <= 0.6.1, as upper version bounds

Using ghc-prim <= 0.6.1 is somewhat dodgy from a PVP point of view, as it makes it awkward to support new minor releases of ghc-prim. Let's instead use < 0.7, which is the idiomatic way of expressing PVP-compliant upper version bounds.