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/runtime/parachains/src/runtime_api_impl/mod.rs

@@ -25,5 +25,6 @@
 //! 1. Bump the version of the stable module (e.g. `v2` becomes `v3`)
 //! 2. Move methods from `vstaging` to `v3`. The new stable version should include all methods from
 //!    `vstaging` tagged with the new version number (e.g. all `v3` methods).
-pub mod v5;
+
+pub mod v7;
 pub mod vstaging;