contrib/org-mac-link.el: fix Mail.app link compatibility with macOS 11

* org-mac-link.el (org-mac-message-open): Replace angular brackets
with encoded version (< to %3C and > to %3E) so macOS 11 Big Sur can
open the message in mail.app.

macOS 11 does not open the mail link with angular brackets any more.
This fix is backwards compatible.  I have tested it on macOS 10.15.7.

TINYCHANGE

contrib/lisp/org-mac-link.el

@@ -912,7 +912,7 @@ selected items in DEVONthink Pro Office and make link(s) out of it/them."
   "Visit the message with MESSAGE-ID.
 This will use the command `open' with the message URL."
   (start-process (concat "open message:" message-id) nil
-                 "open" (concat "message://<" (substring message-id 2) ">")))
+                 "open" (concat "message://%3C" (substring message-id 2) "%3E")))
 
 (defun org-as-get-selected-mail ()
   "AppleScript to create links to selected messages in Mail.app."