Migrate polkadot-primitives to v6 (#1543)

- Async-backing related primitives are stable `primitives::v6`
- Async-backing API is now part of `api_version(7)`
- It's enabled on Rococo and Westend runtimes

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>

polkadot/node/network/gossip-support/src/lib.rs

@@ -452,7 +452,7 @@ where
 				// match void -> LLVM unreachable
 				match message {
 					Versioned::V1(m) => match m {},
-					Versioned::VStaging(m) => match m {},
+					Versioned::V2(m) => match m {},
 				}
 			},
 		}