Drop admin.system.keys on CSRS downgrade from 3.6 fcv to 3.4 fcv

The admin.system.keys collection should be dropped on downgrade csrs to v3.4 (dependent on SERVER-29637)

Here is the scenario we want to prevent:
1. Upgrade csrs to v3.6.
2. Downgrade csrs to v3.4.
3. Either add a new node to csrs or maybe one of the secondaries became too stale and need to do initial sync.
4. One of the secondary will now claim to be 'replicated' after the sync without copying the keys.
5. Upgrade csrs to v3.6. One of the secondary will not contain the existing keys even though it is supposed to be 'caught up'.

Note: this can be an issue because of selective initial sync

A test should be added to verify this scenario does not happen.