diagnostics: 1.8.4-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `diagnostics` to `1.8.4-0`:

- upstream repository: https://github.com/ros/diagnostics.git
- release repository: https://github.com/ros-gbp/diagnostics-release.git
- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.11`
- previous version for package: `1.8.3-0`