Rust 1.71 lints (#4503)

## Issue Addressed

N/A

## Proposed Changes

Add lints for rust 1.71

[3789134](3789134ae2) is probably the one that needs most attention as it changes beacon state code. I changed the `is_in_inactivity_leak ` function to return a `ArithError` as not all consumers of that function work well with a `BeaconState::Error`.

slasher/tests/backend.rs

@@ -1,4 +1,4 @@
-#![cfg(all(feature = "lmdb"))]
+#![cfg(feature = "lmdb")]
 
 use slasher::{config::MDBX_DATA_FILENAME, Config, DatabaseBackend, DatabaseBackendOverride};
 use std::fs::File;