We should stop generating the last LTS constant as kFullyDowngradedTo...}, since this will cause redness upon branching. For example, before the 6.0 release, we referenced the 6.0 FCV enum as kVersion_6_0. However after branching, if we want to immediately add the new git tag to upgrade FCV constants, the last LTS will be kFullyDowngradedTo_6_0. This will cause the old kVersion_6_0 references to break, leading to unavoidable redness on evergreen. In order to immediately upgrade after branching without redness, we should stop generating last LTS as kFullyDowngradedTo...