Alerting: Make Unified Alerting enabled by default for those who do not use legacy alerting (#42200)

* update AlertingEnabled and UnifiedAlertingSettings.Enabled to be pointers
* add a pseudo migration to fix the AlertingEnabled and UnifiedAlertingSettings.Enabled if the latter is not defined
* update the default configuration file to make default value for both 'enabled' flags be undefined

Misc
* update Migrator to expose DB engine. This is needed for a ualert migration to access the database while the list of migrations is created.
* add more verbose failure when migrations do not match

Co-authored-by: gotjosh <josue@grafana.com>
Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>
Co-authored-by: gillesdemey <gilles.de.mey@gmail.com>

pkg/setting/setting.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/grafana/grafana-aws-sdk/pkg/awsds"
 	"github.com/grafana/grafana-plugin-sdk-go/backend/gtime"
+
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/util"
 
@@ -153,7 +154,7 @@ var (
 	Quota QuotaSettings
 
 	// Alerting
-	AlertingEnabled            bool
+	AlertingEnabled            *bool
 	ExecuteAlerts              bool
 	AlertingRenderLimit        int
 	AlertingErrorOrTimeout     string
@@ -914,9 +915,6 @@ func (cfg *Cfg) Load(args CommandLineArgs) error {
 	if err := readAlertingSettings(iniFile); err != nil {
 		return err
 	}
-	if err := cfg.ReadUnifiedAlertingSettings(iniFile); err != nil {
-		return err
-	}
 
 	explore := iniFile.Section("explore")
 	ExploreEnabled = explore.Key("enabled").MustBool(true)
@@ -1377,7 +1375,11 @@ func (cfg *Cfg) readFeatureToggles(iniFile *ini.File) error {
 
 func readAlertingSettings(iniFile *ini.File) error {
 	alerting := iniFile.Section("alerting")
-	AlertingEnabled = alerting.Key("enabled").MustBool(true)
+	enabled, err := alerting.Key("enabled").Bool()
+	AlertingEnabled = nil
+	if err == nil {
+		AlertingEnabled = &enabled
+	}
 	ExecuteAlerts = alerting.Key("execute_alerts").MustBool(true)
 	AlertingRenderLimit = alerting.Key("concurrent_render_limit").MustInt(5)