Fix Rust beta compiler errors (1.77) (#5180)

* Lint fixes

* More fixes for beta compiler.

* Format fixes

* Move `#[allow(dead_code)]` to field level.

* Remove old comment.

* Update beacon_node/execution_layer/src/test_utils/mod.rs

Co-authored-by: João Oliveira <hello@jxs.pt>

* remove duplicate line

beacon_node/execution_layer/src/test_utils/mock_builder.rs

@@ -54,7 +54,8 @@ impl Operation {
 }
 
 #[derive(Debug)]
-struct Custom(String);
+// We don't use the string value directly, but it's used in the Debug impl which is required by `warp::reject::Reject`.
+struct Custom(#[allow(dead_code)] String);
 
 impl warp::reject::Reject for Custom {}