Use JDK21 Platform-wide

[dduportal]

October 2024: JDK11 is end of life and JDK17 will be the minimum requirement for Jenkins weekly in late June 2024.

The Jenkins infra want to stay on top and support the Jenkins community contribution on the JDK plans: as such we want to use JDK21 as a default everywhere.

This issue is an EPIC to track the related work.

• [x] Provide JDK21 to developer early on ci.jenkins.io
  • [x] EA JDK21 - https://github.com/jenkins-infra/helpdesk/issues/3709
  • [x] GA and further upgrades JDK21 - https://github.com/jenkins-infra/helpdesk/issues/3783 | https://github.com/jenkins-infra/helpdesk/issues/4058
• [x] #4124
• [ ] #4123
• [ ] #4121
• [ ] #4126
• [ ] #4125
• [ ] #4127

[dduportal]

Update: we have WiP issues related to JDK21 stability and/or memory usage:

• https://github.com/jenkins-infra/helpdesk/issues/3890
• https://github.com/jenkins-infra/helpdesk/issues/3874

[dduportal]

Update:

• RPU JDK baseline was changed in https://github.com/jenkins-infra/repository-permissions-updater/pull/3953 but it breaks the trusted.ci execution. Issue to be created to track the fix
  • Incident issue: https://github.com/jenkins-infra/helpdesk/issues/4122
  • Related change issue (sub task): https://github.com/jenkins-infra/helpdesk/issues/4124