MDL-80676 core: Deprecate \action_menu_link::$instance

\action_menu_link::$instance is only being used to provide unique IDs
for the <span> tag that contains the text within the action menu link's
<a> tag. This <span> tag's ID is then being used for the action menu
link's <a> tag's aria-labelledby attribute which is totally unnecessary
given that the <span> tag is already within the action menu link and
already serves as the default label for the <a> tag.

lib/outputcomponents.php

@@ -4954,6 +4954,8 @@ class action_menu_link extends action_link implements renderable {
 
     /**
      * The number of instances of this action menu link (and its subclasses).
+     *
+     * @deprecated since Moodle 4.4.
      * @var int
      */
     protected static $instance = 1;
@@ -4982,7 +4984,6 @@ public function __construct(moodle_url $url, ?pix_icon $icon, $text, $primary =
      */
     public function export_for_template(renderer_base $output) {
         $data = parent::export_for_template($output);
-        $data->instance = self::$instance++;
 
         // Ignore what the parent did with the attributes, except for ID and class.
         $data->attributes = [];

lib/upgrade.txt

@@ -34,6 +34,7 @@ information provided here is intended especially for developers.
   - `\core\deprecation::is_deprecated(example::class);`
   - `\core\deprecation::emit_deprecation_if_present([self::class, 'some_method']);`
 * Added missing deprecation for PARAM_CLEANFILE which was deprecated in Moodle 2.0.
+* \action_menu_link::$instance has been deprecated as it is no longer used.
 
 === 4.3 ===