[JENKINS-72157] ensure uptime is independent of system clock

[mawinter69]

use nanoTime to compute time delta.

See JENKINS-72157.

Testing done

• start Jenkins check uptime via script console
• change system time
• verify uptime is still correct

Proposed changelog entries

• JENKINS-72157, ensure uptime is independent of system clock

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

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[daniel-beck]

Thanks!

Could be used to simplify/make consistent https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/jenkins/telemetry/impl/Uptime.java but that seems unnecessary (and would technically change the start time definition).

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!