SERVER-41696 removes the 'ns' field from indexes in 4.4 upgrade and SERVER-41697 removed the logic from the latest mongod binary completely for renaming the 'ns' field of the index. We should continue to rename the 'ns' field of the index when on FCV4.2 or we can fail to startup on downgrade due to this fassert during startup recovery.

Steps to reproduce:
1. Create a collection db_one on a 4.2 mongod binary

2 Restart using master binary on FCV4.2 and rename a collection from db_one to db_two

• The 'ns' field of the index will not be renamed

3. restart again as a 4.2 binary

• On startup, we fail to validate the index because the collection name 'db_two' does not match the index 'ns' field 'db_one'