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

beacon_node/http_api/src/lib.rs

@@ -2840,7 +2840,7 @@ pub fn serve<T: BeaconChainTypes>(
                             let is_live =
                                 chain.validator_seen_at_epoch(index as usize, request_data.epoch);
                             api_types::LivenessResponseData {
-                                index: index as u64,
+                                index,
                                 epoch: request_data.epoch,
                                 is_live,
                             }
@@ -2876,7 +2876,7 @@ pub fn serve<T: BeaconChainTypes>(
         .and_then(
             |sysinfo, app_start: std::time::Instant, data_dir, network_globals| {
                 blocking_json_task(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_bn(
                         sysinfo,
                         data_dir,