Fix guarded-patterns: replace instanceof pattern matching with Java 8 compatible old code

[Copilot]

The old code in guarded-patterns used instanceof pattern matching with a binding variable (shape instanceof Circle c), which requires Java 16+ (JEP 394) and is incompatible with the oldLabel: "Java 8" annotation.

Change

• content/language/guarded-patterns.yaml: Replaced the old code's instanceof pattern matching syntax with a Java 8 compatible version using an explicit cast.

// Before (Java 16+ only)
if (shape instanceof Circle c) {
    if (c.radius() > 10) {
        return "large circle";
    } else {
        return "small circle";
    }
} else {
    return "not a circle";
}

// After (Java 8 compatible)
if (shape instanceof Circle) {
    Circle c = (Circle) shape;
    if (c.radius() > 10) {
        return "large circle";
    } else {
        return "small circle";
    }
} else {
    return "not a circle";
}

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[brunoborges]

The old code must be Java 8 compatible @copilot

[Copilot]

The old code must be Java 8 compatible @copilot

Done in bd016a6 — reverted oldLabel back to "Java 8" and replaced the old code with Java 8 compatible syntax (explicit instanceof check + (Circle) shape cast instead of the pattern matching binding variable).