Update fullcalendar (library behind calendar compoenent)

[ankush]

• We use v3.x, v6.x is latest version.
• Also remove vendoring of library

[ankush]

@rutwikhdev 👀

[ankush]

https://fullcalendar.io/docs/v4/upgrading-from-v3
https://fullcalendar.io/docs/v5/upgrading-from-v4
https://fullcalendar.io/docs/v6/upgrading-from-v5

[michal-bee]

@ankush @rutwikhdev It would be fantastic to add a config option that includes a license code - so that users who have paid for a license can use the FullCalendar's premium features - the Resource Timeline and Resource Time grid in ERPNext.