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/types/Cargo.toml

@@ -8,9 +8,10 @@ authors = [
 edition = { workspace = true }
 
 [features]
-default = ["legacy-arith"]
-# Allow saturating arithmetic on slots and epochs. Enabled by default, but deprecated.
-legacy-arith = []
+default = []
+# Enable +, -, *, /, % operators for Slot and Epoch types.
+# Operations saturate instead of wrapping.
+saturating-arith = []
 sqlite = ["dep:rusqlite"]
 arbitrary = [
     "dep:arbitrary",