MDL-76974 user: deprecate old method of setting user preferences.

The `M.util.set_user_preference` method (plus accompanying helpers)
are now deprecated, in favour of the `core_user/repository` module
to achieve the same.

Update current implementation to proxy calls to the new API ensuring
backwards compatibility, while emitting copious developer debugging.

lib/javascript-static.js

@@ -195,27 +195,16 @@ M.util.CollapsibleRegion.prototype.icon = null;
  * user_preference_allow_ajax_update from moodlelib.php to tell Moodle that
  * the udpate is allowed, and how to safely clean and submitted values.
  *
- * @param String name the name of the setting to udpate.
- * @param String the value to set it to.
+ * @param {String} name the name of the setting to update.
+ * @param {String} value the value to set it to.
+ *
+ * @deprecated since Moodle 4.3.
  */
 M.util.set_user_preference = function(name, value) {
-    YUI().use('io', function(Y) {
-        var url = M.cfg.wwwroot + '/lib/ajax/setuserpref.php?sesskey=' +
-                M.cfg.sesskey + '&pref=' + encodeURI(name) + '&value=' + encodeURI(value);
+    Y.log('M.util.set_user_preference is deprecated. Please use the "core_user/repository" module instead.', 'warn');
 
-        // If we are a developer, ensure that failures are reported.
-        var cfg = {
-                method: 'get',
-                on: {}
-            };
-        if (M.cfg.developerdebug) {
-            cfg.on.failure = function(id, o, args) {
-                alert("Error updating user preference '" + name + "' using ajax. Clicking this link will repeat the Ajax call that failed so you can see the error: ");
-            }
-        }
-
-        // Make the request.
-        Y.io(url, cfg);
+    require(['core_user/repository'], function(UserRepository) {
+        UserRepository.setUserPreference(name, value);
     });
 };