cbbackupmgr should use syncronous DCP for 6.6.0

The Couchbase source will automatically determine whether or not to stream vBuckets sequentially depending on the Couchbase server version (it can also be forced using an environment variable). When introduced, cloud integration was intended for 7.0.0 (Cheshire-cat) however this is no longer the case. We need to change this version check to look for 6.6.0 instead of 7.0.0 otherwise cloud backups will use the wrong DCP client.