mgr/dashboard: Enable read only users to read again

mgr/dashboard: Enable read only users to read again

The dashboards Prometheus receiver API needed to receive a POST from the
frontend, but POSTs aren't allowed by default by any read only
users. As a result the receiver API call had thrown a 403 error which
redirected the user to the 403 error page. Now you can get the last
notifications via GET. This prevents the redirection for read only
users, as a result they can get the last notifications and also
see all other allowed pages again.

Fixes: https://tracker.ceph.com/issues/39086
Signed-off-by: Stephan Müller <smueller@suse.com>
(cherry picked from commit 35a75d088963043c77830865bc943e7d8c30f7e7)