Update customize-package-emacs-version-alist

* lisp/org.el (customize-package-emacs-version-alist): Map Org 9.6 to
Emacs 29.1.

Emacs 28.1 is nearing release.  The earliest Emacs version that Org
9.6 would be synced with is 29.1.

lisp/org.el

@@ -4993,7 +4993,8 @@ The following commands are available:
 		   ("9.2" . "27.1")
 		   ("9.3" . "27.1")
 		   ("9.4" . "27.2")
-		   ("9.5" . "28.1")))
+		   ("9.5" . "28.1")
+		   ("9.6" . "29.1")))
 
 (defvar org-mode-transpose-word-syntax-table
   (let ((st (make-syntax-table text-mode-syntax-table)))