Don't use deprecated unload-hook variables in Gnus

Don't use deprecated unload-hook variables in Gnus

* lisp/gnus/gnus-registry.el (gnus-registry-unload-hook):
* lisp/gnus/spam-stat.el (spam-stat-unload-hook):
* lisp/gnus/spam.el (spam-unload-hook): Rename functions from this...

* lisp/gnus/gnus-registry.el (gnus-registry-unload-function):
* lisp/gnus/spam-stat.el (spam-stat-unload-function):
* lisp/gnus/spam.el (spam-unload-function): ...to this.  Don't set the
corresponding variables, always return nil, update callers, and leave
old name as obsolete function aliases.
Ref: https://lists.gnu.org/r/emacs-devel/2025-02/msg00294.html

(cherry picked from commit ceef9025944fd3d51122f913ed9a74a259db55f7)