fix non compilable code on eclipse compiler (#5375)

legacyDescriptors has no idea what type of Descriptor it has and is
using a raw Descrriptor.
legacyDescriptors.computeIfAbsent can then only return keys of the type
that are in the map - and this is a raw type - so no generic is possible
in the return of getLegacyDescriptors()

fixes the eclipse error ->
Type mismatch: cannot convert from
CopyOnWriteArrayList<ExtensionComponent<Descriptor>> to
CopyOnWriteArrayList<ExtensionComponent<Descriptor<T>>>