All sharded cluster upgrade procedure mention 6.0

All the documentation pages for upgrade-sharded-cluster procedure , under the Re-enable the balancer section , includes the "/includes/extracts/4.2-changes-start-balancer-autosplit.rst" file that has specific details about the autosplitter changes for 6.0.

This information should be included only in the 6.0 upgrade procedure and not for previous version (5.0, 4.4, 4.0). For instance a user upgrading from 4.4 to 5.0 doesn't care about changes we introduced in 6.0