[JENKINS-73302] Restore margins around setup wizard alert messages

[MarkEWaite]

[JENKINS-73302] Restore margins around setup wizard alert messages

JENKINS-73302 reports that when a user of the setup wizard skips the creation of a user and skips the assignment of the Jenkins URL, the two alert messages no longer have a margin between them.

This adds a top margin to each of the messages so that when two messages are displayed, there will be a margin between them. Follows the same pattern of margin definition that is used elsewhere in Jenkins core.

Testing done

Checked the visual display with 2 messages and with each message alone.

Confirmed that margins were already included in the "Manage Jenkins" page when the old data admin monitor and the deprecated plugins admin monitor are both displayed. This change is not needed for the "Manage Jenkins" page, only for the setup wizard.

Before 2.459

After 2.459 and before the change

After the change

Single warning alert (user creation skipped)

Single warning alert (Jenkins URL skipped)

Proposed changelog entries

• Restore margins around setup wizard alert messages (regression in 2.459).

Proposed upgrade guidelines

N/A

Submitter checklist

Beta Give feedback

1. The Jira issue, if it exists, is well-described.
2. 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.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. 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).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mawinter69

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

Maintainer checklist

Beta Give feedback

1. There are at least two (2) approvals for the pull request and no outstanding requests for change.
2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. 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).
6. If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!