Update sabre/dav

[phsc84]

The update can now be performed, because Nextcloud 11 will only support PHP >=5.6.

Currently used sabre/dav version: 3.0.9
Latest sabre/dav version: 3.2

reference to ownCloud: owncloud/core#26115

[phsc84]

Some memory optimizations have been done in sabre/dav:
https://github.com/fruux/sabre-dav/pull/886

Maybe these are relevant for Nextcloud as well.

[phsc84]

sabre/dav 3.2 will also help with issues like owncloud/contacts#488