MDL-55071 forms: Add upgrade.txt note about submitlink

We are deprecating submitlink because it was added a long time ago
and never used in Moodle. We can't test it or maintain it.

Part of MDL-55071

lib/upgrade.txt

@@ -3,6 +3,7 @@ information provided here is intended especially for developers.
 
 === 3.2 ===
 
+* MForms element 'submitlink' has been deprecated.
 * Node.js versions >=4 are now required to run grunt.
 * JQuery has been updated to 3.1.0. JQuery migrate plugins are no longer shipped - please read
   https://jquery.com/upgrade-guide/3.0/ and update your javascript.