deps: promote futures to workspace dependency

Promote `futures` to workspace dependency for easier management.

Pull-Request: libp2p#5329.

Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["peer-to-peer", "libp2p", "networking"]
 categories = ["network-programming", "asynchronous"]
 
 [dependencies]
-futures = "0.3.29"
+futures = { workspace = true }
 libp2p-core = { workspace = true }
 libp2p-identity = { workspace = true, features = ["peerid"] }
 libp2p-swarm = { workspace = true }