Update FullCalendar library to V5.x, create 8.x-3.x?

Problem/Motivation

FullCalendar library 5.0 was released on 2020/06/21. Since old major releases do not get security or bug fixes, this module needs to be updated to the version 5.x branch, possibly requiring a 8.x-3.x branch. The current library version is 5.3.2 as of 2020/09/06.
https://github.com/fullcalendar/fullcalendar/blob/master/CHANGELOG.md
https://fullcalendar.io/blog/2020/02/changes-in-the-upcoming-v5

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes