Use unnamed variable for unused try-with-resources resource on Java 21+

[timtebeek]

• Fixes UseTryWithResources names the resource variable even when it is not referenced in the try body, producing an unused-variable warning that fails -Werror builds #927

UseTryWithResources always emitted a named resource variable when converting try/finally to try-with-resources, even when the resource is never referenced inside the try body. On codebases compiled with -Xlint:all -Werror (which promotes the unused warning to an error), the rewritten code no longer compiles.

Change

When the resource variable is not referenced anywhere in the try body and the target Java version is >= 21, the recipe now emits the unnamed variable _ (JEP 443 preview in Java 21, finalized as JEP 456 in Java 22) instead of a named variable:

// before
Scope scope = openScope();
try {
} finally {
    scope.close();
}

// after (Java 21+)
try (Scope _ = openScope()) {
}

For targets below Java 21 (where _ is not available) the existing behavior is preserved — the named variable is kept. Resources that are read in the body keep their name on all versions.

The renamed variable's JavaType.Variable and identifier fieldType are updated alongside the printed name so type validation stays consistent.

Tests

• unusedResourceUsesUnnamedVariableOnJava21 — unused resource becomes _ on Java 21
• unusedResourceKeepsNameBelowJava21 — unused resource keeps its name below Java 21
• resourceUsedInBodyKeepsNameOnJava21 — resource read in the body keeps its name on Java 21