$majorityConfig is a custom write concern used for reconfig that is supposed to ensure that the current config, C, must be installed on at least a majority of voting nodes in C. This prevents a scenario where a config prior to C can independently elect a new primary and commit a write.

However, we are incorrectly calculating the config majority here. We should just be using _majorityVoteCount. That code calculates the majority incorrectly for any set >= 4 members (for 4 and 5 nodes, majority is calculated as 2 instead of 3).