fix deprecation warnings

[klkvsk]

Some analyzers (e.g. Symfony DebugClassLoader) advise to add @return annotation along with #[ReturnTypeWillChange]:

"User Deprecated: Method "ArrayObject::unserialize()" might add "void" as a native return type declaration in the future. Do the same in child class "Diff\DiffOp\Diff\Diff" now to avoid errors or add an explicit @return annotation to suppress this message."