[JENKINS-69787] Use (x) icon for dismissing admin monitors

[Jayesh45-master]

Fixes #69787

Summary

Replace text-based Dismiss buttons in Administrative Monitor banners with a close (×) icon, aligning the UI with Jenkins design conventions and improving visual consistency.

This change is limited to the web UI and does not alter any backend behavior.

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Testing done

• Built Jenkins from source using:
  mvn -DskipTests clean install
• Ran Jenkins locally using:
  java -jar war/target/jenkins.war --enable-future-java
• Triggered multiple Administrative Monitors (e.g. Reverse Proxy warning, Root URL not set, Security warnings).
• Verified that:
• The dismiss action is rendered as an (×) icon instead of a text button.
• Hovering over the icon shows the Dismiss tooltip.
• The button includes an appropriate aria-label for accessibility.
• Clicking the icon successfully dismisses the admin monitor banner.

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Before

Admin Monitor banners displayed a blue Dismiss text button.

After

Admin Monitor banners display a close (×) icon for dismissing.

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Proposed changelog entries

• Use a close (×) icon instead of a text button for dismissing administrative monitor warnings.

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Proposed changelog category

/label web-ui, skip-changelog

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Proposed upgrade guidelines

N/A

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Submitter checklist

• The issue is well-described.
• The change is limited to UI behavior and does not alter functionality.
• Manual testing performed and documented.
• No new public APIs introduced.
• UI changes do not violate CSP rules.
• No upgrade steps required.

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Desired reviewers

@jenkinsci/core-pr-reviewers

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[NotMyFault]

Duplicate of #25964