ros_realtime: 1.0.21-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `ros_realtime` to `1.0.21-0`:

- upstream repository: https://github.com/ros/ros_realtime.git
- release repository: https://github.com/TheDash/ros_realtime-release.git
- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.12`
- previous version for package: `1.0.17-0`