Fix guarded-patterns: revert oldLabel to Java 8, use Java 8 compatible old code

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>
Agent-Logs-Url: https://github.com/javaevolved/javaevolved.github.io/sessions/3b716d37-be8d-4d05-8f2c-416583048b5c

Author: Copilot

content/language/guarded-patterns.yaml

@@ -5,12 +5,13 @@ title: "Guarded patterns with when"
 category: "language"
 difficulty: "intermediate"
 jdkVersion: "21"
-oldLabel: "Java 16"
+oldLabel: "Java 8"
 modernLabel: "Java 21+"
 oldApproach: "Nested if"
 modernApproach: "when Clause"
 oldCode: |-
-  if (shape instanceof Circle c) {
+  if (shape instanceof Circle) {
+      Circle c = (Circle) shape;
       if (c.radius() > 10) {
           return "large circle";
       } else {