Update to Fullcalendar library v6

So I just merged 5.x into 6.x resolved merge conflicts and added support for v6 which includes: modification of few libraries and replacing few deprecated options with new ones. Also added the ability to use once instead of jquery.once.

+1 for this patch. We have been using it on a large D10 project for the past few months with success. It helped us meet our accessibility initiatives.

@mattdanger are you using this patch on the latest 5.x version of the module, or have you switched to 6.x that has the upgrade to v5 of the library installed - https://www.drupal.org/project/fullcalendar_view/issues/3085884 → ?