multi_cluster.js has no helpers for upgrading individual nodes in the config server, which makes it quite roundabout to get the extra coverage for mixed versions as all examples in the codebase use the helper that upgrades all nodes at the same time. This behavior is not of particular interest to the query team, however it may prompt errors in specific edge cases. For example in SERVER-95169 upgrading a multiple node config server would make the server fail to elect a primary and fail the test, whereas using a single-node config server would work perfectly fine. This was hard to debug as the default number of nodes is 3, and as seen in SERVER-97370 some tests use incorrect definitions for the amount of config server nodes.