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/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: