[CI] Port CI fixes from the 1.2.0 branch

[hcho3]

• Publish GPU-enabled JAR artifacts to xgboost-maven-repo S3 bucket. The artifacts are named xgboost4j-gpu and xgboost4j-spark-gpu.
• Use a machine with multiple GPUs (4 NVIDIA T4s) to run the unit tests for xgboost4j-spark-gpu.
• Fix a unit test on CLI, to handle RC versions

[codecov-commenter]

Codecov Report

Merging #6050 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6050   +/-   ##
=======================================
  Coverage   79.10%   79.10%           
=======================================
  Files          12       12           
  Lines        3044     3044           
=======================================
  Hits         2408     2408           
  Misses        636      636           

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