lisp/org.el: Allow hiding #+SUBTITLE: keyword via `org-hidden-keywords'

* lisp/org.el: Allow users to include 'subtitle in
`org-hidden-keywords' to hide #+SUBTITLE: keyword.

This way #+SUBTITLE is treated like similar keywords for title, date,
e-mail, and author.

etc/ORG-NEWS

@@ -12,6 +12,12 @@ Please send Org bug reports to mailto:emacs-orgmode@gnu.org.
 
 * Version 9.5 (not yet released)
 ** New options and settings
+*** Option ~org-hidden-keywords~ now also applies to #+SUBTITLE:
+
+The option ~org-hidden-keywords~ previously applied
+to #+TITLE:, #+AUTHOR:, #+DATE:, and #+EMAIL:.  Now it can also be
+used to hide the #+SUBTITLE: keyword.
+
 *** New formatting directive ~%L~ for org-capture
 
 The new ~%L~ formatting directive contains the bare link target, and

lisp/org.el

@@ -3569,10 +3569,11 @@ lines to the buffer:
 For example, a value \\='(title) for this list makes the document's title
 appear in the buffer without the initial \"#+TITLE:\" part."
   :group 'org-appearance
-  :version "24.1"
+  :package-version '(Org . "9.5")
   :type '(set (const :tag "#+AUTHOR" author)
 	      (const :tag "#+DATE" date)
 	      (const :tag "#+EMAIL" email)
+	      (const :tag "#+SUBTITLE" subtitle)
 	      (const :tag "#+TITLE" title)))
 
 (defcustom org-custom-properties nil