Arvados

As reported by gradle --warning-mode all:

> Configure project :                                                                                                          
The maven plugin has been deprecated. This is scheduled to be removed in Gradle 7.0. Please use the maven-publish plugin instea
d. Consult the upgrading guide for further information: https://docs.gradle.org/6.9.4/userguide/upgrading_version_5.html#legacy
_publication_system_is_deprecated_and_replaced_with_the_publish_plugins

This does indeed block an upgrade to Gradle 7+. Migrate this plugin as documented, then upgrade the version of Gradle we use in test-in-docker.sh.