docs: add note to not recommended Traditional Rules

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -221,6 +221,10 @@ and the new classes (**Strict Rules**) have ``CodeIgniter\Validation\StrictRules
 Traditional Rules
 -----------------
 
+.. important:: Traditional Rules exist only for backward compatibility. Do not
+    use them in new projects. Even if you are already using, we recommend that
+    you switch to Strict Rules.
+
 .. warning:: When validating data that contains non-string values, such as JSON data, it is recommended to use **Strict Rules**.
 
 The **Traditional Rules** implicitly assume that string values are validated,