Use API4 for 5.24 support

CRM/Groupadmin/Form/Edit.php

@@ -21,6 +21,7 @@ class CRM_Groupadmin_Form_Edit extends CRM_Core_Form {
 
     $this->assign('groupTitle', civicrm_api3('Group', 'getvalue', ['id' => $this->groupID, 'return' => 'title']));
 
+    /* Join not supported until 5.27 - https://github.com/civicrm/civicrm-core/commit/16f5a13d62017fa30190a477b9f580b4eabd23a2
     $groupAdminContacts = \Civi\Api4\GroupAdminContact::get()
       ->addSelect('contact_id', 'contact.display_name')
       ->setJoin([
@@ -29,6 +30,12 @@ class CRM_Groupadmin_Form_Edit extends CRM_Core_Form {
       ->addWhere('group_id', '=', $this->groupID)
       ->setCheckPermissions(FALSE)
       ->execute();
+    */
+
+    $groupAdminContacts = civicrm_api3('GroupAdminContact', 'get', [
+      'return' => ['contact_id', 'contact.display_name'],
+      'group_id' => $this->groupID,
+    ])['values'];
 
     foreach ($groupAdminContacts as $groupAdminContact) {
       $adminContacts[$groupAdminContact['contact_id']] = $groupAdminContact['contact.display_name'];