fix the 'add' module page javascript failing on Moodle 4.3+

[usqfowlerj]

Checklist

• the contributor license agreement is signed — the CLA URL is a 404 now, and I had signed it under a different github username anyway
• commit message follows commit guidelines
• screenshots are included showing significant UI changes — N/A
• documentation is changed or added — N/A

Description of change

From Moodle 4.3, YUI module moodle-core_availability-form now expects to find elements with ids fitem_id_availabilityconditionsjson and availabilityconditions-loading.

[edalex-ian]

Thanks for raising this @usqfowlerj , this unfortunately fell through our review processes. We'll schedule in some review and see if we can get this in.