fix(modelmigration): restore v28 import

Author: 0xMax42

modelmigration/migrations.go

@@ -32,6 +32,7 @@ import (
 	"gitea.dev/modelmigration/v1_7"
 	"gitea.dev/modelmigration/v1_8"
 	"gitea.dev/modelmigration/v1_9"
+	"gitea.dev/modelmigration/v28"
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"