org: Prefer standard version format accepted by `version-to-list'

* lisp/org.el: Rename Org version on development branch from "9.6-dev"
to "9.6-pre".  The latter is accepted by `version-to-list'.

See https://orgmode.org/list/87edvzbniw.fsf@gnu.org

lisp/org.el

@@ -9,7 +9,7 @@
 ;; URL: https://orgmode.org
 ;; Package-Requires: ((emacs "25.1"))
 
-;; Version: 9.6-dev
+;; Version: 9.6-pre
 
 ;; This file is part of GNU Emacs.
 ;;