Clean up after next `cyclonedx-maven-plugin` upgrade

Once CycloneDX/cyclonedx-maven-plugin#419 is solved and published, we can remove the [`process-sbom`](https://github.com/apache/logging-parent/blob/78bbf5ef2c3305e41ba312604b3577037c907426/pom.xml#L642) execution and add the external reference to the plugin configuration.