PrincipalsBackend: update interface to SabreDAV 2.1.0, add tests

Add a simple constructor test to check whether we can instantiate this object
without throwing syntax errors.
AuthBasicBackend: add simple test for constructor
CardDavBackend: add simple test for constructor
VCard\Parser: add simple test for constructor
composer.json: hammer SabreDAV version down to 2.1.x

The ~2.0 notation was permitting upgrades to 2.1, which broke things. Use
stricter version notation to prevent this in the future.
Update ChangeLog, release version 0.23