After PM-2091, we will not generate any FCV constants older than lastLTS. This will need to be modified so that we can temporarily have references to these older FCVs after upgrading FCV constants. We can potentially do this by adding a new value to releases.yml that determines whether or not to generate old FCV constants.

This ticket will let us delay removing old FCV constants until a later period after upgrading, making the window between branch cut and updating FCV constants much smaller.