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

[zdtsw]

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...

[zdtsw]

@steelhead31 this might be useful when 20 is out, and we need decommission 19, plus some cleanup work for 18 too