Fix guarded-patterns oldLabel: Java 8 → Java 16

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>
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Author: Copilot

content/language/guarded-patterns.yaml

@@ -5,7 +5,7 @@ title: "Guarded patterns with when"
 category: "language"
 difficulty: "intermediate"
 jdkVersion: "21"
-oldLabel: "Java 8"
+oldLabel: "Java 16"
 modernLabel: "Java 21+"
 oldApproach: "Nested if"
 modernApproach: "when Clause"