Usage of the soon-to-be-deprecated Logger.warn/1 API.

The library uses the soon-to-be-deprecated Logger.warn/1 API.

```
==> persistent_ets
Compiling 3 files (.ex)
     warning: Logger.warn/1 is deprecated. Use Logger.warning/2 instead
     │
 165 │     Logger.warn("Unknown message received by #{inspect(self())}: #{inspect(other)}")
     │            ~
     │
     └─ lib/table_manager.ex:165:12: PersistentEts.TableManager.handle_info/2
```