Tidy eth1/deposit contract logging (#3397)

## Issue Addressed

Fixes an issue identified by @remyroy whereby we were logging a recommendation to use `--eth1-endpoints` on merge-ready setups (when the execution layer was out of sync).

## Proposed Changes

I took the opportunity to clean up the other eth1-related logs, replacing "eth1" by "deposit contract" or "execution" as appropriate.

I've downgraded the severity of the `CRIT` log to `ERRO` and removed most of the recommendation text. The reason being that users lacking an execution endpoint will be informed by the new `WARN Not merge ready` log pre-Bellatrix, or the regular errors from block verification post-Bellatrix.

beacon_node/client/src/notifier.rs

@@ -441,14 +441,14 @@ fn eth1_logging<T: BeaconChainTypes>(beacon_chain: &BeaconChain<T>, log: &Logger
 
                 warn!(
                     log,
-                    "Syncing eth1 block cache";
+                    "Syncing deposit contract block cache";
                     "est_blocks_remaining" => distance,
                 );
             }
         } else {
             error!(
                 log,
-                "Unable to determine eth1 sync status";
+                "Unable to determine deposit contract sync status";
             );
         }
     }