Use `as_nanos` in `Ord` impl for `DateTime` (#2656)

## Motivation and Context
This PR adresses on [feedback from my previous
PR](https://github.com/awslabs/smithy-rs/pull/2653#pullrequestreview-1404515899).


## Description
- use `as_nanos` in `Ord` impl for `DateTime` instead of manual impl.

## Testing
- add proptest that checks that `Ord` impl matches RFC 3339 comparison.

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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