Update FullCalendar to 6.x

Problem/Motivation

We should update the FullCalendar package. We are currently using vesion 4.4.2. The current version is 6.x. Since this module is forked from fullcalendar_view, we could probably get a lot of the needed changes from there: 📌 Update to Fullcalendar library v6 Needs review

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes