SERVER-65809 introduced the `transitionFCVFromAndTo` map to associate the various upgrade/downgrade FCV constants with the expected start and finish FCV version constants. `getTransitionFCVFromandTo` uses the map to retrieve the expected start and finish FCV versions given an upgrade/downgrade FCV constant. The method currently returns the FCV versions by value but const-qualifies them, which is causing clang-tidy to fail.

The function should return a const ref to the map entry instead.