[JENKINS-73845] Fix OperatingSystemEndOfLifeAdminMonitor endOfLifeDate displayed on first warning day

[Dohbedoh]

See JENKINS-73845. On the first day the warning should be displayed, the EOL date displayed was the default 2099-12-31.

Testing done

• Change the end-of-life-data.json with:

  {
    "pattern": "Ubuntu.* 20.04.*",
    "endOfLife": "2025-04-24"
  },

• Build Jenkins WAR
• Validate that it shows the monitor and the correct EOL date

Proposed changelog entries

• Fix End of Life Operating System monitor that shows 2099-12-31 on the first day a warning should be displayed.

Proposed upgrade guidelines

N/A

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[Dohbedoh]

Not 100% of the result of the test case added. I believe as long as the running agent is linux (i.e. has an /etc/os-release) it should be fine. Maybe the test should be skipped for Windows though..
Maybe I should inject a file: for the test JSON.

[timja]

Thanks!

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!