Create a checklist for how to decommission old jdk version when we do not support any more

For example, when we start to release jdk19, we do not need to support build jdk18 anymore.
Certain things are good to do:
- disable nightly pipeline and weekly pipeline, this can save disk space and jenkins resource. set `disableJob = true` in configurations/jdkXX.groovy (see 15 as example)
- update nightly status check https://github.com/adoptium/ci-jenkins-pipelines/blob/master/tools/nightly_build_and_test_stats.groovy
- retired old version https://github.com/adoptium/ci-jenkins-pipelines/blob/master/pipelines/build/regeneration/build_pipeline_generator.groovy so we do not keep generate it again.
- manually delete nightly and weekly pipeline and all their downstream jobs (double check: this might need jenkins admin permission)
- remove workspace on jenkins "worker" node for this jdk version(double check to confirm)
- update pr-test job https://github.com/adoptium/ci-jenkins-pipelines/blob/master/pipelines/build/prTester/pr_test_pipeline.groovy
- update documentation: readme, comments etc
- remove GH action test https://github.com/adoptium/temurin-build/blob/master/.github/workflows/build.yml#L22
- more...

Review the listed pipeline files, including configurations/jdkXX.groovy, tools/nightly_build_and_test_stats.groovy, build_pipeline_generator.groovy, pr_test_pipeline.groovy, and temurin-build/.github/workflows/build.yml. Compare the JDK 15 example and existing decommissioning steps, then document a complete checklist covering pipeline, workspace, job, documentation, and permission checks; done means the checklist is clear and repeatable.

Written by the indexing model from the issue text.