Upgrade to oldest Jenkins still recommended

[reftel]

The Jenkins documentation states that one should not use versions no longer supportrted by the update center, and that the oldest LTS that is, is 2.426.3. Therefore, it´s probably a good idea to upgrade to that.

Description

See JENKINS-XXXXX.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description.
• Appropriate unit or acceptance tests or explanation to why this change has no tests.
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description.
• Reviewed the code.
• Verified that the appropriate tests have been written or valid explanation given.
• If applicable, tested by installing this plugin on the Jenkins instance.