[JENKINS-69787] Use symbol-x icon for dismiss buttons in admin monitors

[adityasah104]

Replace symbol-close with symbol-x icon in repeatableDeleteButton to improve UX consistency with standard dismiss patterns.

Fixes #18321

This change updates the dismiss button icon in admin monitors from symbol-close to symbol-x to align with standard UI patterns for dismiss/close actions. This addresses JENKINS-69787 and improves visual consistency across the Jenkins UI.

Testing done

• Built Jenkins successfully with mvn clean install
• Manually verified the dismiss button displays the X icon correctly in admin monitors
• Confirmed no functional regression - dismiss/delete behavior works as expected
• Tested in "Execute shell" dialog and other repeatable delete button contexts
• Visual inspection confirms improved consistency with standard dismiss patterns

Screenshots (UI changes only)

Before

See screenshots in JENKINS-69787 showing the symbol-close icon.

After

This PR changes symbol-close to symbol-x, replacing the close icon with an X icon for better UX consistency.

Proposed changelog entries

• Use X icon for dismiss buttons in admin monitors to improve UX consistency

Proposed changelog category

/label rfe,web-ui

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/ux

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[github-actions]

Missing required label for changelog. Requires at least 1 of: bug, developer, dependencies, internal, localization, major-bug, major-rfe, rfe, regression-fix, removed, skip-changelog. Found: spam.

You can add the required label by adding a comment with the following text: /label <category>