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/operation_pool/src/attestation.rs

@@ -49,7 +49,7 @@ impl<'a, T: EthSpec> AttMaxCover<'a, T> {
         let indices = get_attesting_indices::<T>(committee.committee, &fresh_validators).ok()?;
         let fresh_validators_rewards: HashMap<u64, u64> = indices
             .iter()
-            .map(|i| *i as u64)
+            .copied()
             .flat_map(|validator_index| {
                 let reward = base::get_base_reward(
                     state,