Update to SCALE 1.2.0 (#5113)

This updates `parity-scale-codec` to `1.2.0`, which includes multiple
performance improvements and a fix that bounds the capacity of a vector
at decoding.

substrate/client/rpc-api/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/paritytech/substrate/"
 description = "Substrate RPC interfaces."
 
 [dependencies]
-codec = { package = "parity-scale-codec", version = "1.0.0" }
+codec = { package = "parity-scale-codec", version = "1.2.0" }
 derive_more = "0.99.2"
 futures = { version = "0.3.1", features = ["compat"] }
 jsonrpc-core = "14.0.3"