Address Clippy 1.73 lints (#4809)

## Proposed Changes

Fix Clippy lints enabled by default in Rust 1.73.0, released today.

validator_client/src/attestation_service.rs

@@ -193,7 +193,7 @@ impl<T: SlotClock + 'static, E: EthSpec> AttestationService<T, E> {
             .into_iter()
             .fold(HashMap::new(), |mut map, duty_and_proof| {
                 map.entry(duty_and_proof.duty.committee_index)
-                    .or_insert_with(Vec::new)
+                    .or_default()
                     .push(duty_and_proof);
                 map
             });

validator_client/src/duties_service/sync.rs

@@ -163,7 +163,7 @@ impl SyncDutiesMap {
 
         committees_writer
             .entry(committee_period)
-            .or_insert_with(CommitteeDuties::default)
+            .or_default()
             .init(validator_indices);
 
         // Return shared reference