Clippy lints for rust 1.66 (#3810)

## Issue Addressed
Fixes the new clippy lints for rust 1.66

## Proposed Changes

Most of the changes come from:
- [unnecessary_cast](https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast)
- [iter_kv_map](https://rust-lang.github.io/rust-clippy/master/index.html#iter_kv_map)
- [needless_borrow](https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow)

## Additional Info

na

validator_client/src/doppelganger_service.rs

@@ -441,7 +441,7 @@ impl DoppelgangerService {
         }
 
         // Get a list of indices to provide to the BN API.
-        let indices_only = indices_map.iter().map(|(index, _)| *index).collect();
+        let indices_only = indices_map.keys().copied().collect();
 
         // Pull the liveness responses from the BN.
         let request_epoch = request_slot.epoch(E::slots_per_epoch());
@@ -971,16 +971,16 @@ mod test {
         LivenessResponses {
             current_epoch_responses: detection_indices
                 .iter()
-                .map(|i| LivenessResponseData {
-                    index: *i as u64,
+                .map(|&index| LivenessResponseData {
+                    index,
                     epoch: current_epoch,
                     is_live: false,
                 })
                 .collect(),
             previous_epoch_responses: detection_indices
                 .iter()
-                .map(|i| LivenessResponseData {
-                    index: *i as u64,
+                .map(|&index| LivenessResponseData {
+                    index,
                     epoch: current_epoch - 1,
                     is_live: false,
                 })

validator_client/src/http_api/mod.rs

@@ -331,7 +331,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
         .and(signer.clone())
         .and_then(|sysinfo, app_start: std::time::Instant, val_dir, signer| {
             blocking_signed_json_task(signer, move || {
-                let app_uptime = app_start.elapsed().as_secs() as u64;
+                let app_uptime = app_start.elapsed().as_secs();
                 Ok(api_types::GenericResponse::from(observe_system_health_vc(
                     sysinfo, val_dir, app_uptime,
                 )))

validator_client/src/initialized_validators.rs

@@ -472,7 +472,7 @@ impl InitializedValidators {
 
     /// Iterate through all voting public keys in `self` that should be used when querying for duties.
     pub fn iter_voting_pubkeys(&self) -> impl Iterator<Item = &PublicKeyBytes> {
-        self.validators.iter().map(|(pubkey, _)| pubkey)
+        self.validators.keys()
     }
 
     /// Returns the voting `Keypair` for a given voting `PublicKey`, if all are true:

validator_client/src/key_cache.rs

@@ -104,7 +104,7 @@ impl KeyCache {
         let file = File::options()
             .read(true)
             .create_new(false)
-            .open(&cache_path)
+            .open(cache_path)
             .map_err(Error::UnableToOpenFile)?;
         serde_json::from_reader(file).map_err(Error::UnableToParseFile)
     }