XDCR - should prevent explicit/migration mode until source cluster is fully upgraded

When performing rolling upgrades, esp when a cluster is running in mixed mode where some nodes are < 7.0 and some are 7.0, XDCR does not prevent the users from activating collection-related modes/toggles.

In the interest of safety, XDCR should enforce implicit mapping and not allow users to edit anything collection-related until it is confirmed that source cluster is fully upgraded.

Also, it may make sense to check target cluster compatibility before allowing collection-related feature in replication settings to be set.