Context: We have the concept of uninitialized FCV i.e. a node still has not created the FCV document. For example, when launching a mongod --replSet rs0 --dbpath /some/empty/folder and before the node is added to a replica set (e.g. via rs.add()or rs.initiate() then all commands run under uninitialized FCV.

In the code uninitialized FCV is represented by the kUnsetDefaultLastLTSBehavior FCV constant.

Issues:

• The "kUnset" prefix doesn't make it immediately clear that this is uninitialized FCV.
• The "DefaultLastLTSBehavior" suffix is misleading because checking a feature flag is enabled during uninitialized FCV does not behave like lastLTS, rather it fail (see SERVER-115905). Instead we provide two methods to decide whether to behave like lastLTSFCV or latestFCV.

Proposal: Rename kUnsetDefaultLastLTSBehavior to kUninitialized.