image_transport_plugins: 1.8.21-1 in 'groovy/release.yaml' [bloom]

Increasing version of package(s) in repository `image_transport_plugins`:
- previous version: `1.8.20-0`
- new version: `1.8.21-1`
- distro file: `groovy/release.yaml`
- bloom version: `0.4.1`