[CI] Use Ubuntu 18.04 LTS in JVM CI, because 19.04 is EOL (#5537)
[CI] Use Ubuntu 18.04 LTS in JVM CI, because 19.04 is EOL (#5537)


dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost