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.

• Provide JDK21 to developer early on ci.jenkins.io
  • EA JDK21 - Add JDK21 support for developers to pre-built and try #3709
  • GA and further upgrades JDK21 - track the new jdk21 version from adoptium #3783 | JDK patch upgrade campaign (April 2024) #4058
• Add JDK21 agents (build) #4124
• Move controllers to JDK21 (runtime) #4123
• Move agents to JDK21 (runtime) #4121
• Switch default JDK to 21 for pipeline libraries #4126
• Switch default JDK to 21 for build tools #4125
• Switch agent (java home) to JDK21 default #4127

[dduportal]

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

• Possible change to ci.jenkins.io's linux agents causes core test failure #3890
• Intermittent out of memory for Java 21 builds of Jenkins core on ci.jenkins.io #3874

[dduportal]

Update:

• RPU JDK baseline was changed in [JENKINS-73253] Build and test with Java 21 repository-permissions-updater#3953 but it breaks the trusted.ci execution. Issue to be created to track the fix
  • Incident issue: [RPU] builds stuck in trusted.ci.jenkins.io #4122
  • Related change issue (sub task): Add JDK21 agents (build) #4124