Rename `xref-pop-marker-stack` to `xref-go-back` (bug#38797)

Rename `xref-pop-marker-stack` to `xref-go-back` (bug#38797)

It is the natural name after the addition of `xref-go-forward`.
The old name is retained as an alias.

* lisp/progmodes/xref.el (xref-pop-marker-stack): Rename.
(xref-go-forward, xref-quit-and-pop-marker-stack)
(xref-find-definitions):
* lisp/menu-bar.el (menu-bar-goto-menu):
* lisp/progmodes/etags.el (find-tag-marker-ring, pop-tag-mark):
* lisp/progmodes/prog-mode.el (prog-context-menu):
* doc/emacs/maintaining.texi (Looking Up Identifiers):
* etc/NEWS:
Use the new name.