UMLR — Classifier::ownedTemplateSignature needs to subset Element::ownedElement

UML composite properties must subset Element::ownedElement.
Classifier::ownedTemplateSignature doesn't.

Note that issue 12244 suggested making a change to replace
the ownedElement subset with a redefinition of another
property. By 2.5 the change had already been made, so
the issue was closed no change in ballot 1. However it was
never the correct thing to do as the previously resolved
general composite issue 14926 pointed out.