org-agenda: Replace 29.1 version keywords with package-version

* lisp/org-agenda.el (org-agenda-block-separator):
(org-agenda-time-grid):
(org-agenda-current-time-string): Replace version keyword with
package-version keyword.

Prefer package-version so that the help buffer shows

  This variable was introduced, or its default value was changed, in
  version 9.6 of the Org package that is part of Emacs 29.1.

rather than

  This variable was introduced, or its default value was changed, in
  version 29.1 of Emacs.

lisp/org-agenda.el

@@ -147,7 +147,7 @@ If it is a character, it will be repeated to fill the window width.
 If nil the separator is disabled.  In `org-agenda-custom-commands' this
 addresses the separator between the current and the previous block."
   :group 'org-agenda
-  :version "29.1"
+  :package-version '(Org . "9.6")
   :type '(choice
 	  (const :tag "Disabled" nil)
 	  (character)
@@ -1571,7 +1571,7 @@ times that have a grid line.
 The fourth item is a string placed after the grid times.  This
 will align with agenda items."
   :group 'org-agenda-time-grid
-  :version "29.1"
+  :package-version '(Org . "9.6")
   :type
   '(list
     (set :greedy t :tag "Grid Display Options"
@@ -1600,7 +1600,7 @@ will align with agenda items."
     "now - - - - - - - - - - - - - - - - - - - - - - - - -")
   "The string for the current time marker in the agenda."
   :group 'org-agenda-time-grid
-  :version "29.1"
+  :package-version '(Org . "9.6")
   :type 'string)
 
 (defgroup org-agenda-sorting nil