Default alerting to Alertmanager API v2

Alertmanager API v2 is available for more than 2 years now, there's no
reason to not use it by default.

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

jsonnet/kube-prometheus/components/prometheus.libsonnet

@@ -251,6 +251,7 @@ function(params) {
           namespace: p.config.namespace,
           name: 'alertmanager-' + p.config.alertmanagerName,
           port: 'web',
+          apiVersion: 'v2',
         }],
       },
       securityContext: {