Make issue warning banner non dismissible on milestone page

Rinku Crequested to merge
gitlab-community/gitlab-org/gitlab:rc_fix_gitlab-org/gitlab#536280 into master

What does this MR do and why?

Make issue warning banner non dismissible on milestone page

References

#536280 (closed)

Screenshots or screen recordings

Before After
Screenshot 2026-02-28 at 18.57.53.png Screenshot 2026-02-28 at 18.58.56.png

How to set up and validate locally

  1. Checkout this branch and run gdk locally
  2. Go to milestones page from project or from homepage milestone's menu
  3. Open any single milestone that has more than 500 issues or add not after if if not display_issues_count_warning?(``@milestone``) on line no 3 of app/views/shared/milestones/_issues_tab.html.haml to emulate showing of issue warning banner temporarily.
  4. Notice the banner doesn't have close button
  5. Compare to master branch where it does have close button which doesn't work

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