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/roadmap/implementers-guide/src/node/backing/prospective-parachains.md

@@ -122,7 +122,7 @@ prospective validation data. This is unlikely to change.
 
 ### Outgoing
 
-- `RuntimeApiRequest::StagingParaBackingState`
+- `RuntimeApiRequest::ParaBackingState`
   - Gets the backing state of the given para (the constraints of the para and
     candidates pending availability).
 - `RuntimeApiRequest::AvailabilityCores`