Alerting: Remove ngalert feature toggle and introduce two new settings for enabling Grafana 8 alerts and disabling them for specific organisations (#38746) (#39793)

* Remove `ngalert` feature toggle

* Update frontend

Remove all references of ngalert feature toggle

* Update docs

* Disable unified alerting for specific orgs

* Add backend tests

* Apply suggestions from code review

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Disabled unified alerting by default

* Ensure backward compatibility with old ngalert feature toggle

* Apply suggestions from code review

Co-authored-by: gotjosh <josue@grafana.com>
(cherry picked from commit 012d4f0905)

pkg/tests/api/alerting/api_prometheus_test.go

@@ -21,8 +21,9 @@ import (
 
 func TestPrometheusRules(t *testing.T) {
 	dir, path := testinfra.CreateGrafDir(t, testinfra.GrafanaOpts{
-		EnableFeatureToggles: []string{"ngalert"},
-		DisableAnonymous:     true,
+		DisableLegacyAlerting: true,
+		EnableUnifiedAlerting: true,
+		DisableAnonymous:      true,
 	})
 
 	grafanaListedAddr, store := testinfra.StartGrafana(t, dir, path)
@@ -265,8 +266,9 @@ func TestPrometheusRules(t *testing.T) {
 
 func TestPrometheusRulesPermissions(t *testing.T) {
 	dir, path := testinfra.CreateGrafDir(t, testinfra.GrafanaOpts{
-		EnableFeatureToggles: []string{"ngalert"},
-		DisableAnonymous:     true,
+		DisableLegacyAlerting: true,
+		EnableUnifiedAlerting: true,
+		DisableAnonymous:      true,
 	})
 
 	grafanaListedAddr, store := testinfra.StartGrafana(t, dir, path)