chore: update default JENKINS_VERSION to 2.534

[lemeurherve]

This PR updates the default JENKINS_VERSION used during builds (not publication) to Jenkins 2.534, the first version enabling Java 25 support.

Extracted from:

• feat: add JDK25 images #2146

Refs:

• https://github.com/jenkinsci/jenkins/releases/tag/jenkins-2.534
• Enable Java 25 support jenkins#11148

Testing done

make test

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[lemeurherve]

The s390x agent appears unavailable. Build n°1 got it passing with success: https://ci.jenkins.io/job/Packaging/job/docker/view/change-requests/job/PR-2163/1/pipeline-overview/?selected-node=55
All other stages succeeded in build n°2 and build n°3.

Force-squashing this PR.