[JENKINS-76306] Fix placeholder page, remove inline JS

[daniel-beck]

See JENKINS-76306.

Discovered while looking for CSP violations in preparation for jenkinsci/jenkins#11269, and this one seems to have slipped through the cracks last year.

Testing done

mvn hpi:run, then navigate to http://localhost:8080/jenkins/monitoring/nodes?part=graph&graph=usedMemory
Then run in script console (different tab):

Jenkins.get().pluginManager.getPlugin('monitoring').plugin.filter.nodesCollector.lastJavaInformationsList.clear()
System.setProperty('javamelody.nodes-monitoring-disabled', "true")

Then refresh the first tab.

Previously: Raw HTML shown, or file downloaded (see JENKINS issue).
Now:

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 demonstrate the feature works or the issue is fixed

[evernat]

thank you Daniel