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>

cumulus/parachains/integration-tests/emulated/common/src/lib.rs

@@ -64,7 +64,7 @@ decl_test_relay_chains! {
 			Hrmp: kusama_runtime::Hrmp,
 		}
 	},
-	#[api_version(6)]
+	#[api_version(7)]
 	pub struct Westend {
 		genesis = westend::genesis(),
 		on_init = (),
@@ -79,7 +79,7 @@ decl_test_relay_chains! {
 			Balances: westend_runtime::Balances,
 		}
 	},
-	#[api_version(5)]
+	#[api_version(7)]
 	pub struct Rococo {
 		genesis = rococo::genesis(),
 		on_init = (),
@@ -94,7 +94,7 @@ decl_test_relay_chains! {
 			Balances: rococo_runtime::Balances,
 		}
 	},
-	#[api_version(5)]
+	#[api_version(7)]
 	pub struct Wococo {
 		genesis = rococo::genesis(),
 		on_init = (),