[jvm-packages] Clean up CI

[hcho3]

Closes https://github.com/dmlc/xgboost/issues/10879 Closes https://github.com/dmlc/xgboost/issues/7262

• Properly handle shaded JARs
• Remove xgboost4j-tester; fold integration tests as part of the test suite, so that mvn test would suffice.
• Add test coverage for all demos. Update the demos so that they can run without failing.
• Remove unused Maven profiles
• Update the CI pipelines to deploy the shaded JARs (xgboost4j-spark / xgboost4j-spark-gpu)

Note. I will submit a follow-up PR to update dev/prepare_jvm_release.py.

New workflow:

1. mvn deploy -Pdefault,release-to-s3, to deploy all CPU packages (xgboost4j, xgboost4j-example, xgboost4j-spark, xgboost4j-flink).
2. mvn clean to clean all artifacts
3. mvn install -Pgpu, to build xgboost4j and xgboost4j-spark-gpu with CUDA enabled and to install them locally.
4. mvn deploy -Pgpu,release-to-s3 -pl xgboost4j-spark-gpu to deploy xgboost4j-spark-gpu only.

[hcho3]

@wbo4958 The PR is now ready for review.

See the full build log at https://buildkite.com/organizations/xgboost/pipelines/xgboost-ci/builds/7247/jobs/01930a4e-98e2-4dad-bc10-ab2fdd58f376/log

[trivialfis]

Remove xgboost4j-tester

Is there an alternative? We have no test for xgboost jvm demos.

[hcho3]

I'll bring back the integration tests.

[hcho3]

Marking as draft until I bring back the integration tests.

[hcho3]

@wbo4958 I moved the integration tests so that mvn test -pl xgboost4j-example will run them. I discovered that some demos were left out in the previous version of integration tests. Now all demos have test coverage.

[trivialfis]

I assume we can close this one as well https://github.com/dmlc/xgboost/issues/7262 ?