Update Substrate deps to stable2603-rc2.

Author: Neopallium

Cargo.lock

@@ -118,7 +118,7 @@ scale-info = { workspace = true, default-features = false, features = [
     "derive",
 ] }
 bounded-collections = { workspace = true, default-features = false }
-sp-io = { git = "https://github.com/PolymeshAssociation/polkadot-sdk", branch = "polymesh-v8-stable2512", default-features = false, optional = true }
+sp-io = { git = "https://github.com/PolymeshAssociation/polkadot-sdk", branch = "polymesh-v8-stable2603-rc2", default-features = false, optional = true }
 
 # serde support
 serde = { workspace = true, default-features = false, features = [