Enhancement: Log correct oCIS version if available

oCIS has introduced a new `productversion` field to announce it's correct version while maintaining a fake 10.x.x version in the `versionstring` field to keep clients compatible. We're using the new productversion field when it exists and use versionstring as a fallback. Thus the backend product information prints the correct oCIS version now.

https://github.com/owncloud/ocis/pull/3805
https://github.com/owncloud/web/pull/7045
