Enumerate and remove Execution-related FeatureCompatibilityVersion 4.0-dependent code and tests
Remove WiredTiger specific upgrade/downgrade interface from kv engine layer.
After 4.2 check_unique_indexes.js shouldn't rely on a specific FCV version
Remove FCV upgrade test from setFCV_collmod_transaction_upgrade
Unique index that gets rebuilt as part of startup recovery/repair is using data format version as 6 or 8 and NOT 11 or 12.
4.2 nodes that use auth and initial sync will fail to start in 4.4