Perf: add extra index to notification table (#32395)

Index SQL: `CREATE INDEX u_s_uu ON notification(user_id, status,
updated_unix);`

The naming follows `action.go` in the same dir.

I am unsure which version I should add SQL to the migration folder, so I
have not modified it.

Fix #32390

models/migrations/migrations.go

@@ -366,6 +366,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(306, "Add BlockAdminMergeOverride to ProtectedBranch", v1_23.AddBlockAdminMergeOverrideBranchProtection),
 		newMigration(307, "Fix milestone deadline_unix when there is no due date", v1_23.FixMilestoneNoDueDate),
 		newMigration(308, "Add index(user_id, is_deleted) for action table", v1_23.AddNewIndexForUserDashboard),
+		newMigration(309, "Improve Notification table indices", v1_23.ImproveNotificationTableIndices),
 	}
 	return preparedMigrations
 }