feat: add Windows 2025 images for docker-ssh-agent

[lemeurherve]

This PR adds Windows 2025 images (nanoserver & windows-server-core variants).
It also set the agent to use for Windows 2022+ images to a Windows Server Core host as it can build both 2022 and 2025 images (not the 20219 though).

In draft until trusted.ci.jenkins.io has Windows 2025 VM agent templates.

Ref:

• Add support for Windows 2025 agents jenkins-infra/helpdesk#4791 (comment)

Testing done

• Manual validation builds on an external VM
• Replay: https://ci.jenkins.io/job/Packaging/job/docker-ssh-agent/job/master/852/replay/diff
• CI

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