Fix `CheckedCallable.unchecked()` for return type

Currently the `CheckedCallable.unchecked()` returns `Runnable` instead of `Callable<T>` and its implementation does not return value from the target `call()` invocation.
Apparently this is a copy/paste artifact from the `CheckedRunnable`.

To avoid breaking changes, we are going to introduce another method and deprecate this one.
Will be removed in the next `6.5.x` version.