Disable legacy-arith by default in consensus/types (#8695)

Currently, `consensus/types` cannot build with `no-default-features` since we use "legacy" standard arithmetic operations.


  - Remove the offending arithmetic to fix compilation.
- Rename `legacy-arith` to `saturating-arith` and disable it by default.


Co-Authored-By: Mac L <mjladson@pm.me>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1665,7 +1665,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                         let validator_index = *validator_index as usize;
                         committee_cache.get_attestation_duties(validator_index)
                     })
-                    .collect();
+                    .collect::<Result<Vec<_>, _>>()?;
 
                 Ok((duties, dependent_root))
             },