Use "utf8mb4" for MySQL by default (#25432)

TBH, I don't see much difference from `Remove "CHARSET" config option
for MySQL, always use "utf8mb4"` #25413

Close #25413

services/forms/user_form.go

@@ -27,7 +27,6 @@ type InstallForm struct {
 	DbPasswd string
 	DbName   string
 	SSLMode  string
-	Charset  string `binding:"Required;In(utf8,utf8mb4)"`
 	DbPath   string
 	DbSchema string