Use new #[contractevent] macro

In https://github.com/soroswap/aggregator/tree/feat/protocol-23
```
warning: use of deprecated method `soroban_sdk::events::Events::publish`: use the #[contractevent] macro on a contract event type
  --> aggregator/src/event.rs:34:10
   |
34 |         .publish(("SoroswapAggregator", symbol_short!("update")), event);
   |          ^^^^^^^


```