Chore: Remove use of deprecated method in AC code (#87541)

* switch from using cfg to using featuremgmt for checking a feature toggle in AC code

* merge test fixes

pkg/services/ngalert/api/api_alertmanager_test.go

@@ -631,7 +631,7 @@ func createSut(t *testing.T) AlertmanagerSrv {
 	}
 	mam := createMultiOrgAlertmanager(t, configs)
 	log := log.NewNopLogger()
-	ac := acimpl.ProvideAccessControl(setting.NewCfg())
+	ac := acimpl.ProvideAccessControl(featuremgmt.WithFeatures())
 	ruleStore := ngfakes.NewRuleStore(t)
 	return AlertmanagerSrv{
 		mam:        mam,