Obsolete gnus-copy-sequence and gnus-last-element

Obsolete gnus-copy-sequence and gnus-last-element

(Bug#28856, Bug#28867)

* lisp/gnus/gnus-range.el: Leave gnus-last-element in place, but
  obsolete it and point to car of last. Remove gnus-copy-sequence and
  define as an obsolete alias for copy-tree.
  (gnus-remove-from-range):
* lisp/gnus/gnus-cache.el (gnus-cache-generate-active):
* lisp/gnus/gnus-sum.el (gnus-summary-insert-articles,
  gnus-select-newsgroup, gnus-summary-read-group-1,
  gnus-select-newsgroup, gnus-update-marks,
  gnus-summary-insert-articles, gnus-summary-insert-new-articles):
  Replace calls in these locations.
* lisp/gnus/gnus-agent.el (gnus-agent-catchup,
  gnus-agent-summary-fetch-series, gnus-category-copy):
* lisp/gnus/gnus-cus.el (gnus-group-customize):
* lisp/gnus/gnus-group.el (gnus-group-edit-group-done,
  gnus-group-make-useful-group):
* lisp/gnus/gnus-score.el (gnus-score-adaptive):
* lisp/gnus/gnus-srvr.el (gnus-server-copy-server): Replace calls in
  these locations.