mgr/dashboard: use border-subtle utility class in overview and tearsheet

mgr/dashboard: use border-subtle utility class in overview and tearsheet

Replace the remaining custom dashboard border classes in the overview alerts card and tearsheet with the shared border-subtle utility classes. Update the alerts-card unit test so it checks the shared utility class name used by the template. Stub Prometheus overview requests in the dashboard a11y Cypress test so the test does not fail on missing metrics in CI.

Fixes: http://tracker.ceph.com/issues/75404
Signed-off-by: Annmool <aydv.267@gmail.com>