Alerting: Change rule migration to be based on feature flag (#33792)

makes it so the feature flag can be turned on off, and the migration will be cleared and rerun. All existing NG alert rules, configuration settings, etc are removed when disabling the feature flag.
for https://github.com/grafana/alerting-squad/issues/142

Co-authored-by: Sofia Papagiannaki <sofia@grafana.com>

pkg/services/sqlstore/migrator/migrator.go

@@ -6,6 +6,7 @@ import (
 
 	_ "github.com/go-sql-driver/mysql"
 	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/setting"
 	"github.com/grafana/grafana/pkg/util/errutil"
 	_ "github.com/lib/pq"
 	_ "github.com/mattn/go-sqlite3"
@@ -17,6 +18,7 @@ type Migrator struct {
 	Dialect    Dialect
 	migrations []Migration
 	Logger     log.Logger
+	Cfg        *setting.Cfg
 }
 
 type MigrationLog struct {
@@ -28,12 +30,13 @@ type MigrationLog struct {
 	Timestamp   time.Time
 }
 
-func NewMigrator(engine *xorm.Engine) *Migrator {
+func NewMigrator(engine *xorm.Engine, cfg *setting.Cfg) *Migrator {
 	mg := &Migrator{}
 	mg.x = engine
 	mg.Logger = log.New("migrator")
 	mg.migrations = make([]Migration, 0)
 	mg.Dialect = NewDialect(mg.x)
+	mg.Cfg = cfg
 	return mg
 }
 
@@ -168,6 +171,16 @@ func (mg *Migrator) exec(m Migration, sess *xorm.Session) error {
 	return nil
 }
 
+func (mg *Migrator) ClearMigrationEntry(id string) error {
+	sess := mg.x.NewSession()
+	defer sess.Close()
+	_, err := sess.SQL(`DELETE from migration_log where migration_id = ?`, id).Query()
+	if err != nil {
+		return fmt.Errorf("failed to clear migration entry %v: %w", id, err)
+	}
+	return nil
+}
+
 type dbTransactionFunc func(sess *xorm.Session) error
 
 func (mg *Migrator) inTransaction(callback dbTransactionFunc) error {