Get cluster config fails with DocumentNotFound in mixed mode (>= 7.0.2)

See MB-51612 as the client workaround is described: fall back to ns_server REST API when encountered

• I can only bootstrap from the 6.6.3 node; the GET_CONFIG (0xb5) op always returns KEY_NOT_FOUND (0x0001) if I bootstrap from 7.0.3
• If I bootstrap from the 6.6.3 node, the ConfigHandler component (which executes 0xb5 in a round robin fashion) always fails with KEY_NOT_FOUND for the 7.0.3 nodes.
• When in mixed mode (6.6.3 and 7.0.3), even if the bucketcapabilities does not return support for collections, I can call Hello and support collections on a node if that node supports collections.