fix: pin windowsservercore-ltsc2019 to April 2024 release

[lemeurherve]

This PR demonstrates how we could add a WINDOWS_VERSION_DIGEST with the mechanism implemented for jenkinsci#426.

Testing done

### 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 demonstrates feature works or fixes the issue

[lemeurherve]

@dduportal [async] should I open an equivalent PR on jenkinsci/docker-ssh-agent?

[dduportal]

@dduportal [async] should I open an equivalent PR on jenkinsci/docker-ssh-agent?

Yes please: that would be useful!

[lemeurherve]

Opened jenkinsci#433