upgrade db to 5.15 (#11918)

model/version.go

@@ -13,6 +13,7 @@ import (
 // It should be maintained in chronological order with most current
 // release at the front of the list.
 var versions = []string{
+	"5.15.0",
 	"5.14.0",
 	"5.13.0",
 	"5.12.0",