Enable async backing on all testnet system chains (paritytech#2949)

Built on top of paritytech/polkadot-sdk#2826
which was a trial run.

Guide:
https://github.com/w3f/polkadot-wiki/blob/master/docs/maintain/maintain-guides-async-backing.md

---------

Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>
Co-authored-by: Dmitry Sinyavin <dmitry.sinyavin@parity.io>
Co-authored-by: s0me0ne-unkn0wn <48632512+s0me0ne-unkn0wn@users.noreply.github.com>
Co-authored-by: Svyatoslav Nikolsky <svyatonik@gmail.com>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: georgepisaltu <52418509+georgepisaltu@users.noreply.github.com>
(cherry picked from commit 700d5f8)

primitives/chain-bridge-hub-rococo/src/lib.rs

@@ -50,7 +50,7 @@ impl Chain for BridgeHubRococo {
 	}
 
 	fn max_extrinsic_weight() -> Weight {
-		BlockWeights::get()
+		BlockWeightsForAsyncBacking::get()
 			.get(DispatchClass::Normal)
 			.max_extrinsic
 			.unwrap_or(Weight::MAX)