Reintroduce --logfile with deprecation warning (#7723)

Reintroduce the `--logfile` flag with a deprecation warning so that it doesn't prevent nodes from starting. This is considered preferable to breaking node startups so that users fix the flag, even though it means the `--logfile` flag is completely ineffective.

The flag was initially removed in:

- https://github.com/sigp/lighthouse/pull/6339

beacon_node/src/config.rs

@@ -278,7 +278,7 @@ pub fn get_config<E: EthSpec>(
     }
 
     if clap_utils::parse_optional::<u64>(cli_args, "eth1-cache-follow-distance")?.is_some() {
-        warn!("The eth1-purge-cache flag is deprecated");
+        warn!("The eth1-cache-follow-distance flag is deprecated");
     }
 
     // `--execution-endpoint` is required now.