Disable autosplit on config.system.sessions when upgrading the cluster to avoid crashes on shard primaries

Up until v6.0, the chunk splitter is hitting an invariant in case the configured chunk size for a collection is below 1MB. With the changes committed under SERVER-66078, the session collection default chunk size has been decreased to 200KB. Purpose of this ticket is to disable auto-split for the sessions collection in order to avoid hitting the invariant when upgrading from v6.0 to v6.1. This is safe because when upgrading to v6.1, data-size aware balancing will start hence the auto-splitter can be safely disabled.