Issue #83: QA remark: Add the new config for existing sites.

oe_authentication.post_update.php

@@ -0,0 +1,17 @@
+<?php
+
+/**
+ * @file
+ * Post update functions for OE Authentication module.
+ */
+
+declare(strict_types = 1);
+
+/**
+ * Add the 'redirect_user_register_route' module setting.
+ */
+function oe_authentication_post_update_user_register_redirect(): void {
+  \Drupal::configFactory()->getEditable('oe_authentication.settings')
+    ->set('redirect_user_register_route', TRUE)
+    ->save();
+}