Omit fractional seconds from `http-date` format (#2989)

## Motivation and Context
Closes #2831.

## Description
Fractional seconds will still be accepted during parsing, but not
emitted during serialization.

## Testing
```
./gradlew codegen-core:check codegen-client:check codegen-server:check codegen-client-test:check codegen-server-test:check
```

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

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Co-authored-by: Russell Cohen <rcoh@amazon.com>