Run civix upgrade

Trying to install the extension on a test environment with CiviCRM 5.68.0 I get the following error message: Fatal error: Array and string offset access syntax with curly braces is no longer supported in /home/cividev/buildkit/build/wp68/web/wp-content/uploads/civicrm/ext/formfieldlibrary/formfieldlibrary.civix.php on line 246

Running civix upgrade seems to resolve those issues.