Update minor version to 7.8.0 (#22082)

Automatic Merge

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{
+	"7.8.0",
 	"7.7.0",
 	"7.6.0",
 	"7.5.0",