Delete default personal calendar only semi-deactivates it (still takes entries from inivitations, still accessible via DAV)

[tdoerschel]

Steps to reproduce

1. Delete the default calendar (caldav-name "personal")
2. Create a new calender named test
3. recieve an invitation from another user
4. access the (supposedly deleted) personal calendar via remote.php/dav/calendars/Test-ensibo/personal/

Expected behaviour

• deleted calendar should not be accessable anymore
• invitation entries should go to the first active calender owned by the user, or ask which calender (see issue Defining a default calendar #1835 )
• as a workaround, make it easy to restore the personal calendar.

Calendar app 2.3.4

Server configuration

Operating system: (e.g. Debian 8)
Ubuntu 20.04
Web server: (e.g. Apache, Nginx,...)
Apache
Database: (e.g. MariaDB, SQLite or PostgreSQL)
MariaDB 10
PHP version: (e.g. 7.0.3)
8.0.12-FPM
Nextcloud Version: (see admin page, e.g. 17.0.2)
20.2.0
Updated from an older installed version or fresh install:
Fresh Install

P.S. I encountered some import errors on the ICS from an old NC20 server, but only when I imported to the default personal. Importing it to "new Calender" did not show any errors.
That is why I deleted the personal calender. Now I cannot go back :-(

[tcitworld]

Duplicate from nextcloud/server#5040