mgr/dashboard: start using alertmanager v2

mgr/dashboard: start using alertmanager v2

I was looking into sorting the alerts and saw there is an api v2 for
alertmanager which also has an endpoint like `alerts/groups` which might
be something that is useful for us.

Refer: https://github.com/prometheus/alertmanager/blob/main/api/v2/openapi.yaml

Fixes: https://tracker.ceph.com/issues/65070
Signed-off-by: Nizamudeen A <nia@redhat.com>
(cherry picked from commit 6f2d0b14e52885d97fdb48c2e5d7d71201494a8f)

 Conflicts:
	src/pybind/mgr/dashboard/controllers/prometheus.py
  - don't add the endpoint apart from the get_alertgroup one