Use defvaralias for priority variable aliases

* lisp/org.el (org-highest-priority):
(org-lowest-priority):
(org-default-priority): Define as a variable alias, not a function
alias.

When these variables were renamed in e11f50648 (2020-01-30), variable
aliases were defined for the old names, but these were incorrectly
switched to function aliases in e062ca719 (org.el: Use `defalias' for
priority aliases, 2020-02-24).

<https://lists.gnu.org/archive/html/emacs-orgmode/2020-03/msg00083.html>

TINYCHANGE

lisp/org.el

@@ -2383,7 +2383,7 @@ set a priority."
   :group 'org-priorities
   :type 'boolean)
 
-(defalias 'org-highest-priority 'org-priority-highest)
+(defvaralias 'org-highest-priority 'org-priority-highest)
 
 (defcustom org-priority-highest ?A
   "The highest priority of TODO items.
@@ -2394,7 +2394,7 @@ Must be smaller than `org-priority-lowest'."
 	  (character :tag "Character")
 	  (integer :tag "Integer (< 65)")))
 
-(defalias 'org-lowest-priority 'org-priority-lowest)
+(defvaralias 'org-lowest-priority 'org-priority-lowest)
 (defcustom org-priority-lowest ?C
   "The lowest priority of TODO items.
 A character like ?A, ?B, etc., or a numeric value like 1, 2, etc.
@@ -2404,7 +2404,7 @@ Must be higher than `org-priority-highest'."
 	  (character :tag "Character")
 	  (integer :tag "Integer (< 65)")))
 
-(defalias 'org-default-priority 'org-priority-default)
+(defvaralias 'org-default-priority 'org-priority-default)
 (defcustom org-priority-default ?B
   "The default priority of TODO items.
 This is the priority an item gets if no explicit priority is given.