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>

consensus/state_processing/Cargo.toml

@@ -5,9 +5,8 @@ authors = ["Paul Hauner <paul@paulhauner.com>", "Michael Sproul <michael@sigmapr
 edition = { workspace = true }
 
 [features]
-default = ["legacy-arith"]
+default = []
 fake_crypto = ["bls/fake_crypto"]
-legacy-arith = ["types/legacy-arith"]
 arbitrary-fuzz = [
     "types/arbitrary-fuzz",
     "merkle_proof/arbitrary",