Verify that during downgrade commands with readConcern level:snapshot are rejected.

Summary:

• Verify that snapshot read concern is accepted when the cluster is fully upgraded, in both FCV 4.0 and 3.6, then disallowed as soon as mongos is downgraded and at every downgrade stage after.

Design:

• Invert jstests/multiversion/global_snapshot_reads_upgrade_cluster.js to downgrade a cluster instead, and add assertions on the behavior of snapshot reads at each stage.