mgr/dashboard: Fix Grafana Ceph Cluster health status widget

mgr/dashboard: Fix Grafana Ceph Cluster health status widget

The health status widget doesn't show any status because it requires its
query to return a single result. But in case a mgr instance had failed,
it would return more, provided the incident has happened in the
requested time frame.

This is simply an issue of the `instant` switch being disabled for that
widget. As only one mgr instance can ever be providing data at a time,
enabling `instant` completely solves that issue.

Fixes: https://tracker.ceph.com/issues/51212
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>
(cherry picked from commit 4270a13d6c2400162896ce5e4145729615a001e2)