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/protocol/Cargo.toml

@@ -27,6 +27,3 @@ bitvec = "1"
 
 [dev-dependencies]
 rand_chacha = "0.3.1"
-
-[features]
-network-protocol-staging = []