Instructions for updating Jenkins to Java 21: https://www.jenkins.io/doc/book/platform-information/upgrade-java-to-21/

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Upgrade to Java 21 on your Jenkins controller

Stop the Jenkins controller with systemctl stop jenkins.

Install the corresponding Java version with dnf -y install temurin-21-jdk or with the package manager your system uses.

Check the Java version with java -version.

Change the default Java for the system by running update-alternatives --config java and then enter the number that corresponds to Java 21, for example 2 if that is the correct option.

Restart Jenkins with systemctl restart jenkins.