Use Integer::div_floor explicitly

rust-lang/rust#88581 adds several new integer methods as inherent impls. These new methods are a breaking change accepted as a minor change. They already cause build failures with nightly and will eventually cause build failures with stable as well, unless rust-lang changes course. This uses `Integer::div_floor` explicitly to avoid accidentally calling the new nightly methods, same as done for rust-num/num-bigint#218.

rust-runtime/smithy-types/src/instant/mod.rs

@@ -147,7 +147,7 @@ impl Instant {
     /// This is fallible since `Instant` holds more precision than an `i64`, and will
     /// return a `ConversionError` for `Instant` values that can't be converted.
     pub fn to_epoch_millis(&self) -> Result<i64, ConversionError> {
-        let subsec_millis = i64::from(self.subsecond_nanos).div_floor(&(NANOS_PER_MILLI as i64));
+        let subsec_millis = Integer::div_floor(&i64::from(self.subsecond_nanos), &(NANOS_PER_MILLI as i64));
         if self.seconds < 0 {
             self.seconds
                 .checked_add(1)