[CI] Migrate CI pipelines from Jenkins to BuildKite (#8142)

* [CI] Migrate CI pipelines from Jenkins to BuildKite

* Require manual approval

* Less verbose output when pulling Docker

* Remove us-east-2 from metadata.py

* Add documentation

* Add missing underscore

* Add missing punctuation

* More specific instruction

* Better paragraph structure

tests/buildkite/deploy-jvm-packages.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+set -euo pipefail
+
+SPARK_VERSION=3.0.1
+CUDA_VERSION=11.0.3
+
+source tests/buildkite/conftest.sh
+
+if [[ ($is_pull_request == 0) && ($is_release_branch == 1) ]]
+then
+  echo "--- Deploy JVM packages to xgboost-maven-repo S3 repo"
+  tests/ci_build/ci_build.sh jvm_gpu_build docker \
+    --build-arg CUDA_VERSION_ARG=${CUDA_VERSION} \
+    tests/ci_build/deploy_jvm_packages.sh ${SPARK_VERSION}
+fi