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

* update cargo.toml

* use 2.0 in mmmr

substrate/client/consensus/slots/src/aux_schema.rs

@@ -41,7 +41,7 @@ fn load_decode<C, T>(backend: &C, key: &[u8]) -> ClientResult<Option<T>>
 		None => Ok(None),
 		Some(t) => T::decode(&mut &t[..])
 			.map_err(
-				|e| ClientError::Backend(format!("Slots DB is corrupted. Decode error: {}", e.what())),
+				|e| ClientError::Backend(format!("Slots DB is corrupted. Decode error: {}", e)),
 			)
 			.map(Some)
 	}