Correct zstd-jni dependency scope to test only

Beam 2.11.0 introduced a new transitive dep aka zstd-jni. AFAIK it is not needed in most cases so shouldn't be here by default. Also saw it was configured as shadow in the sdk core java module so not sure it is a gradle build bug or intended to be like that but I think sdk-core-java should be cleaned up cause it is now very fast and does not match a lot of usage. Finally this lib being native it is not that sane to bring it by default, in particular with the dockerization happening right now and the goal to have a light container stack (which often implies to not use standard linux as FROM).