The dochub link "http://dochub.mongodb.org/core/3.6-feature-compatibility." is repeated throughout the code.

The help message for setFeatureCompatibilityVersion and cluster setFeatureCompatibilityVersion also contain "3.4" and "3.6" as string constants.

The error message for setting featureCompatibilityVersion also uses string constants for "3.4" and "3.6".

It would be better if all of these string constants were variables so that they could be changed in one place and we would not forget to update any instance when we branch.