CI: remove legacy Concourse CI, and rename concourse to ci

[tuhaihe]

Migrate from Concourse CI to GitHub Actions and remove legacy Greenplum 5/6 support files for Apache Cloudberry compatibility.

Changes:

• Remove .github/workflows/create-release-on-tag.yml
• Rename concourse/ to ci/, keeping only:
  • ci/singlecluster/ (Hadoop test environment)
  • ci/docker/pxf-cbdb-dev/ubuntu/ (Docker Compose + scripts)
• Remove Concourse-specific files:
  • pipelines/, tasks/, scripts/, terraform/, ansible/, settings/
  • docker/{pxf-dev-base,pxf-dev-server,pxf-build-base,docs,sandbox,rpmrebuild}/
• Remove Greenplum 5/6 RPM specs (package/pxf-gp{5,6}.spec)
• Remove obsolete dev scripts:
  • IPA/multinode Hadoop setup
  • Dataproc cluster management
  • create-release-branch.bash, rebuild_pxf.bash, set_up_gpadmin_user.bash
• Remove downloads/README.md
• Update path references in:
  • .github/workflows/pxf-ci.yml
  • README.md, cli/README.md, automation/README.Docker.md, docs/README.md
  • ci/docker/pxf-cbdb-dev/ubuntu/script/{entrypoint.sh,entrypoint_kerberos.sh}

fix #ISSUE_Number

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[tuhaihe]

Hi @my-ship-it @MisterRaindrop @ostinru PTAL, thanks!

[MisterRaindrop]

LGTM