Deprecate util.Immutables

Our util.Immutables class is an attempt to centralize knowledge about when an object can be considered immutable.

Unfortunately this utility has limited use, as JDK11 has a large swathes of immutable collections (List.of() et al.) which cannot be readily identified. Furthermore JDK16's strong encapsulation by default renders reflection-based tricks useless as well.

The class itself is not used anywhere, so let's just give up and ditch it.