Keep unconditional instanceof checks before Java 21 in InstanceOfPatternMatch - #1181

InstanceOfPatternMatch runs on Java 17 sources, but before Java 21 a type pattern must not be unconditional. When the static type of the expression is already a subtype of the tested type, javac rejects the pattern with "expression type is a subtype of pattern type".

ObjectStateMachine<S, E> machine = create();   // ObjectStateMachine implements BeanNameAware
if (machine instanceof BeanNameAware) {
    ((BeanNameAware) machine).setBeanName(beanName);
}

became

if (machine instanceof BeanNameAware aware) {
    aware.setBeanName(beanName);
}

This surfaced while validating the Spring Boot 4.1 migration over open source Spring Boot 3.x repositories: in spring-projects/spring-statemachine (ObjectStateMachineFactory, Java 17) the result no longer compiles with expression type ObjectStateMachine<S,E> is a subtype of pattern type BeanNameAware.

The change leaves an instanceof alone when its expression type is assignable to the tested type and the source file's Java version is below 21 (or unknown). On Java 21 and later, where unconditional patterns are allowed, the recipe behaves as before.

Is this the right change?

• Do we already have it? No. Preserve casts after reassignment in InstanceOfPatternMatch #1074 is about reassigned variables, and Recipe quality triage: mechanical polish (M1–M16) #1115/Recipe quality triage: bugs, wiring and design questions (A1–F3) #1116 don't list this case.
• Could it be declarative? No, it is a guard inside the existing visitor.
• Is it worth it? Yes. Casting to an interface the class already implements is a common legacy idiom, and the result is a compile error.
• Does it fix the problem, or move it? It fixes it: the code is left as it was, which compiles.
• Does it do no harm? It only skips instanceof checks that are always true for non-null values, and only below Java 21.

Tests: InstanceOfPatternMatchTest gains a no-change test on Java 17 (fails without the fix) and a change test on Java 21; all 72 tests in InstanceOfPatternMatchTest pass, as does RemoveInstanceOfPatternMatchTest.