Alerting: fix postgres migration on conflict (#35556) (#35653)

(cherry picked from commit 423474325a)

Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

pkg/services/sqlstore/migrations/ualert/ualert.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"strings"
 
 	pb "github.com/prometheus/alertmanager/silence/silencepb"
 	"xorm.io/xorm"
@@ -217,7 +218,14 @@ func (m *migration) Exec(sess *xorm.Session, mg *migrator.Migrator) error {
 			return err
 		}
 
-		_, err = m.sess.Insert(rule)
+		if strings.HasPrefix(mg.Dialect.DriverName(), migrator.Postgres) {
+			err = mg.InTransaction(func(sess *xorm.Session) error {
+				_, err = sess.Insert(rule)
+				return err
+			})
+		} else {
+			_, err = m.sess.Insert(rule)
+		}
 		if err != nil {
 			// TODO better error handling, if constraint
 			rule.Title += fmt.Sprintf(" %v", rule.Uid)