Update parity-scale-codec to 2.0 (#7994)

* update cargo.toml

* use 2.0 in mmmr

substrate/frame/support/src/event.rs

@@ -713,7 +713,7 @@ mod tests {
 		pub enum TestEventSystemRenamed for TestRuntime2 {
 			system_renamed,
 			event_module<T>,
-			#[codec(index = "5")] event_module2<T>,
+			#[codec(index = 5)] event_module2<T>,
 			event_module3,
 		}
 	}

substrate/frame/support/src/origin.rs

@@ -478,9 +478,9 @@ mod tests {
 	);
 
 	impl_outer_origin!(
-		pub enum OriginIndices for TestRuntime where system = frame_system, system_index = "11" {
+		pub enum OriginIndices for TestRuntime where system = frame_system, system_index = 11 {
 			origin_with_generic<T>,
-			#[codec(index = "10")] origin_without_generic,
+			#[codec(index = 10)] origin_without_generic,
 		}
 	);