Modernize backends dependencies

[tcitworld]

Description

Most of the DAV code relies on Sabre components which aren't very modern. Proper types when implementing interfaces and extending classes would be nicer.

Affected components

• CalDAV and CardDAV backends

To do

• [ ] vObject https://github.com/sabre-io/vobject/pull/562 https://github.com/sabre-io/vobject/pull/586
• [ ] DAV https://github.com/sabre-io/dav/issues/1368
• [ ] Sabre/http

Would be nice to have:

• [ ] Sabre/uri
• [ ] Sabre/xml
• [ ] Sabre/event