Update multiblock metadata (tracking SDK rev 8da42e8fae) (#1036)

`desired_targets` storage item now requires the round index
as a parameter, so this change correctly aligns our code with
the updated storage API in the Polkadot SDK.

See [SDK PR](paritytech/polkadot-sdk#8304).

Author: sigurpol

artifacts/multi_block.scale

@@ -153,8 +153,9 @@ impl BlockDetails {
         let phase = storage
             .fetch_or_default(&runtime::storage().multi_block().current_phase())
             .await?;
+
         let desired_targets = storage
-            .fetch(&runtime::storage().multi_block().desired_targets())
+            .fetch(&runtime::storage().multi_block().desired_targets(round))
             .await?
             .unwrap_or(0);